It's actually a pretty big letdown, but that's not to say it isn't good...
Graphically, the game looks great...there're some fantastic locations and a lot of the enemies are beautifully modelled. Although it seems to run somewhat badly maxed out, fairly weak computers can still run it on lower settings comfortably. The gameplay is great, although slightly repetitive... it's basically a good system, it just needs a little more variety in terms of level design and combos and such. It really makes Oblivion feel stupid in terms of combat. But it's a lot of fun in the end, and it's rather unique too. The storyline isn't actually that bad... there's a fairly predictable plot twist, and the game offers some nice alternate ending. While these are fantstic touches, the game really should have used more of them, and perhaps expanded on the RPG element as well.
Unfortunately though, the game is frustratingly buggy. I've had to make use of quicksave a lot , together with exiting and reloading the game. Very pesky. Also, this game was supposed to have brilliant AI but it's actually kinda crap - enemies are either too stupid in some pars (get stuck around stupid obstacles) or too "intelligent" (fire at one with an arrow from far away, on a different floor, and he finds you instantly). What bothers me about Dark Messiah is that although it's a good game, it could've been a really ****ing awesome game - it does some thing very well, but it really feels as though this game has an incredible potential what was never fully realised, and especially given how good the game is in spite of it's shortcomings, that's a terrible shame. Still, Dark Messiah is a very enjoyable game, and I had a great time playing it. I'll go back to play it again sometime, which is more than I can say for most games. Reccomended.