Haha yes, hopefully to answer your questions:
You play the part of Alex Mercer, who gets transformed into this... thing. He has no memory of how it happened, and wants to find out who did it to him. You get dumped in the middle of a three way war between the human military, the mutant regime, and yourself. You can shape shift into many forms which you unlock as the game progresses, then "purchase" through a points (EP) system. You rack up EP through doing missions, going on rampages and surviving, or locating "landmark" items and "hints" throughout the island of Manhattan.
You play the ultimate neutrality in character design, where you kill EVERYTHING around you, good or bad. You "consume" your victims to absorb their soul/essence in order to heal yourself, take their memories, gain their skills, or shape shift into them!
For awhile I tried not to kill the innocent people, but they get in your way ALL THE TIME! After awhile, you just go berserk and and take down anything stupid enough to come near you... though there are multiple ways of playing the game. You can go for the subtle "blend in with the crowd" method, using your "stealth consume" ability to transform in public undetected, or go balls-out and unleash every ounce of killing power you posses in order to complete your objective. I tend to mix it up a little depending on the mission, but generally I go the KILL EVERYTHING method as it is more FUN!
Yes the game is gory, and yes it is like GTA in the sense of the free-roaming... but that's about where the similarities stop.
All in all, I would recommend this if you are hunting for a game to play. IMO it's probably one of the best, if not THE best game out at the moment in terms of playability, graphics, storyline, ingenuity, and intuitiveness. It does suffer from a mild repetition syndrome as do most games these days, but the missions are varied enough to keep to going, and you can always just do a bit of free-roam to cool off, go nuts, or search for landmarks etc.
I'd give it a solid 9.5/10 (better than Gamespot's 8.5/10)!