I've beaten it on normal ignoring challenges and collectables, and am now half-way through hard trying to get all the collectables and complete many of the challenges. I'm enjoying it a lot and intend to play the hardest difficulty after this (at which time I will ignore the god forsaken collectables and focus on staying alive).
My personal review: the game's got a lot of little flaws (flaky presentation, a really bad story, linear level progression, only a few bosses and only one really good one) but they're superficial flaws mostly. The core gameplay is really fun and the bionic arm mechanics just get better and better as you gain experience with them. The game's biggest sin is lack of basic content. A single play through is only about 8 hours if you don't try to get the relatively pointless "collectables" (tokens that serve no gameplay purpose whatsoever except occasionally to draw your attention to a certain area). Despite the lack of content, the game is worth replaying on harder modes. You take less and less damage forcing you to use the arm more expertly to evade and attack groups of soldiers (and you have a better idea of where you can and can't go). Anyone who was interested in this before the 70% review average ought to at least rent it. Just note that the game likes to kill people who lack skill!