I'm currently loving this game, mainly due to the control scheme. I love the deliberate and cautious feel it has, and it really makes the game.
If you are struggling with it, you either a) need time to adjust or b) just plain suck.
Tip: Don't sit around aiming your weapon as four people close on you. Tap A and back and get the hell out of there. Never allow yourself to be cornered. Movement is more important than shooting, and distance is practically vital.
The inventory system works very well (as one that pauses the game would be hell in Co-op) and with practice makes it fairly easy to select, combine, and use items mid-fight (so long as you follow the tip above). My only gripe is that I seem to be unable to move items into the hotkey slots, but that is pretty minor considering it only takes about a second more to equip whatever it is I'm trying to equip.
And to those of you thinking "That second will get you killed", it won't so long as you aren't trying to hump your target at the same time you are digging around in your pockets.
And don't let Sheva have anything more than the leftovers - she is a pack mule. I personally just gave her a SMG and whatever ammo I can't carry. In fact due to an inventory mix up due to starting the single player game at Chapter 1-2, Sheva was without a weapon and I was able to stock up on, well, everything while taking little to no damage.