Good, but needs balancing
You can grind the early parts of the game, so you're uber-powerful and blow through the rest of it. Might want to implement something to prevent that in the future.
Even when your character is very powerful, he doesn't "feel" powerful. You still feel like the same average joe you started as, except you can do a few more fancy moves.
As for the fancy moves, most are worthless. I went the whole game with nothing but slice (which you start with) and quick slash (2nd attack you gain). Slice was excellent at freezing characters with constant damage (EG: the big dogs in the underground), while quick slash was the "big gun" that could take slow things out with a shot or two. Most other moves either used too much energy to do (for little return on investment), or moved you past your target, making attacking them afterwards difficult. With increased energy recovery maxed out, you can almost quick slash your whole way through constantly.
Some of the skills were just not worth the investment. EG: spend points to increase your life by x%. First level adds 5%. 5 x 100 = 5hp. Whooppee. Forcing the player to max out worthless stuff to get to something better is lame. Some of the skills were about as useful as a tick on a dog's ass, too, like force blade. Pushing enemies out of my attack range is the last thing I want to do. The shield tree sounds interesting, but if I'm blocking it means I'm not defending. And attacking stun-locks the enemy...so no point in blocking.
The skills that were uber powerful were the guardian spirit, damage output increases, crit % increase, damage soak, etc. Eventually you can just plow through guys and shrug off their attacks. The skill that lets you recover life after you kill an enemy sounds good, but it's worthless. Common enemies are not that hard to defeat. The time when you really need health recovery is during the boss battles. But since you're just fighting one boss, there's no enemies around to kill to recover health. The other health skills are more than enough to deal with common enemies.
The stun skill tree seems interesting, but just damaging an enemy quickly stun-locks them anyways. The 3rd skill in the stun tree ... insta-kill an enemy, THAT'S STUNNED ... IF YOU GET A CRIT. There's just too many variables involved for it to be useful often enough to warrant the investment.
Overall, it's interesting, but boring. You just side-scroll past scenery, butcher enemies, level yourself up so you're a walking tank, then mow through the boss before he kills you. The diablo-esque character development is entertaining, but you don't get to feel more powerful after sinking all these points into your character, and you have to max out the previous level (which some are worthless) before you can buy the next level which is what you really want. I really can't believe I'm saying this, because I actually really love passive skill development in characters...I hate having a ton of active skills. I like leveling skills that baby-sit me, not the other way around. But the way this is implemented, it's just not very fulfilling.
The art is very nice, though. Jumping isn't very useful.