I have had this RPG for so long and only recently (okay a few months ago) finally got around to playing it. But now that I have seen it live up to expectations, I wanted to share my impressions/feelings about it.
As usual, I start with the things that are not so good or may be a problem for people.
First of all, the explanation of the story mode character creation system is incomplete/misleading. It is not a big deal as it is easy to deduct that there should be traits handed out during the steps that have nothing listed. It is still inconvenient.
Secondly, I don't see the reason why advantages/disadvantages are limited to combat. Having advantages with normal skill checks as well would offer potential invitations to not simply try something but rather set up the thing to improve your chances, especially since there are no numerical modifiers.
Third, as a personal note, I don't really believe in the default system of using traits. You have six traits. Each positive trait can be used once before it needs to be replenished either by sleeping (which replenishes a single trait) or by suffering the consequences of a negative trait (which also restores a single trait). So, traits are extremely valuable but rare. In the default system, you have to decide whether you invest a trait into a roll beforehand. Which means investing a lot and the possible outcomes do not change thanks to the trait usage for it means that you roll two sets of dice and pick the one you like for your result. The so-called easy mode sounds more reasonable as traits are used to get re-rolls; so you roll first and then decide whether you invest to get a second chance.
Fourth, this combines with third; combat, on the other hand, does not use the default system but rather always easy mode, which kind of runs counter to the idea of a default mode for the game.
Fifth, details like whether you can use multiple traits on a single roll are not dealt with. Similarly, it seems as if there should be a limitation that you can not use normal traits or normal conviction burning to re-roll a result of gruesome death; otherwise the rules for avoiding it would become meaningless.
Sixth, there are tons of random tables, but there are no solo rules for it. Why?
There are a few things that Usurper does not include or that do not work well with it.
Classic dungeon crawls with lots of monsters do not work for the rules. Usurper is designed to have risky combat and the trait system described above means that resources are quite limited. A dungeon full of monsters could probably deal even with experienced pcs as they eventually run out of traits and convictions and are left at the mercy of the dice.
The default setting does not have any spells and no arcane magic users at all. If you are a loyal servant of a god, you may pray for a miracle, but that is left to the fancy of a table to determine in its outcome. Magic in general is nothing for mortals to wield.
That last point may be seen as a positive by people and it is for me. Usurper does not come with a default world you play in, but it comes with the tools of (randomly) generating your own iteration of their default setting of a medieval world of fantasy and horror. As mentioned above, magic is not the providence of men but of gods - and the underdark. The underworld of Usurper is not merely dark and home to monsters, it is a place of madness and chaos and it changes whatever is in it, including adventurers braving it. Monsters are individuals rather than races, although the rules do support non-human pcs and so you could have demihumans or goblinoid races. But things like dragons are supposed to be unique (and can be created randomly).
Add to this the balancing of the system. While player characters are supposed to be competent people with many skills, combat is supposed to be a risky, deadly affair. No matter how experienced your character is, any combat can end with their death. There are also rules for mental trauma, which should be applied in stressful situations which may include the first combat of a character or encountering a new monster. GMs are adviced to play NPCs accordingly, having them flee combant or surrender instead of fighting to the death - until the real monsters appear and break that rule.
The system itself is a narrative system that doesn't feel like it. At its core, you declare your intended action and then roll 1d100 against a table of some 15 possible outcomes including also interruption by a random event. That one table is used for all basic actions except for dealing with gods or combat. You swim across a river, roll on that table. You try to bribe a guard (that's the example), roll on that table. You try to charm a lady, roll on that table. You try to defeat a monster in combat, roll on the combat table. The basic table also has an evil brother, the heroic deed table. It is for cases where your character tries something beyond human power, stretching beyond their limit. While the basic table has a good chance of success, failure is quite likely for a heroic deed and it can be fatal. This is a great tool for improvising high risk actions.
The above may raise two questions:
What about your foes, how do they act? The game is player-facing, that is, only the players roll dice. Their characters either act or react to the actions of others and roll their dice. The GM does not roll dice in task resolution or combat.
What about modifiers? There are none. The game is truely statless having not even fake stats like some non-numerical games have. You have traits and convictions, and you may burn them for re-rolls, that's it. Opponents may have traits that force you to re-roll a good roll and in combat at least, you can get a temporary trait of advantage or disadvantage depending on the situation and on the combatants. That's all there is as far as mechanisms are concerned. While it sounds like a lot of hand-waving, the fact that you are bound to the result of the table makes it rather strict again. You roll (and maybe re-roll) and then you know what happens. While the table describes things in general terms, each entry is quite clear about its message and how you should interpret it for your situation. And the results go beyond simple failure or success giving you a clear direction of where the narration is going now.
It is really interesting as it requires you to think differently about roleplaying and how to evaluate a situation. You have to take a strictly narrative standpoint and examine whether an action is reasonable rather than figuring out which attribute to add to which skill or whatever.
There are also no hit points or sanity points (the game does have rules for mental trauma and insanity); if you goof up in combat, you may get bruised, injured, killed or suffer a gruesome death (which has its own table with interesting results, the most extreme being "So unspeakable that onlookers must roll for Mental Trauma" - and blood and gore everywhere is another entry).
Traits and convictions only give you that powerful special advantage of the re-roll. They are part of what your character is, but they do not limit your character. Even if a character does not have a trait that fits a situation, the character can try to act if it seems plausible, the narration here informs the rules. Because of that, the rules do not have a long list of traits but rather have you freely design your own for your character concept. And since there are no prescribed traits, you are not limited by not having chosen a certain trait, unlike in classic games where lacking a skill means inability or handicap in doing things dependent on that skill. Instead, your character can try anything that your character can reasonably be expected to be able to try (or do a heroic deed going beyond the possible).
The system is extremely elegant and efficient. And it is actually generic. While the random content generators are clearly tuned into the default setting, the core rules, including the tables for combat and task resolution as well as character creation can easily be used for any genre or type of game. Of course, some tables need to be interpreted rather freely and some tables for specific things like damage to spaceships are missing, but the basic core can be used as is. But as I said before, it does not work well with scenarios where resources are used up quickly like dungeon crawls.
In order to try out a tool for creating Cthulhu type stories, I created a modern character using the default story mode character creation system and it worked without any modifications necessary. Unfortunately the tool I tried to use did not live up to expectations, so I didn't get to apply the task resolution or combat rules of Usurper, but they are flexible enough that I would have been able to use them without modification as well.
In addition, it comes with a ton of random tables for generating the world, NPCs (rolling up their aims rather than their looks), cities and their factions, trade goods, dungeon features, monsters, followers, and lots more. There is also an adventure tool for simulating an opposing faction the PCs have to weaken and which may (randomly) strike at them, attacking them or the things dear to them. Of course, there are also tables for personal, local, and regional random events. For those interested in solo play, this game is a perfect fit, and it is surprising that it lacks a GME. And rules for domain play.
So, if you want to play Your character and like a narrative game that still has a solid skeleton while giving you great freedom, I really think you should give this game a try. It is a real gem.