Ambitious and fun...but frustrating, and tedious.
I found myself spending most of the time trying to figure out what certain items were for and why they were even in the game. On one hand the battles were fast and fun, but on the other they seemed a bit pointless. On more than one occasion I found myself running around an area trying to figure out what to do next. I also found myself returning to an area over and over only to find re-spawns and the same old same old.
I liked the almost WOW type of item drops that came after kills, but 90% of the drops seemed worthless and I just ended up bouncing back and forth between drops and shops.
The ability to combine items to create more powerful or different types of weapons was a great idea, but make one mistake and your done for. I often had to resort to saving the game, making my mod, seeing the results and then decide to keep them or load the game and do something else. It took away from the whole concept of the add on's and fell short of delivering solid gameplay in my opinion. I would have to say that I had most of my weaponry by half way through the game and never really found the need, or the drop, to make me change my inventory.
For a FPS RPG it could be worse, but with other games in the genre like Bioshock, this game just doesn't match up.