I agree, it's really easy at some points. Still great fun however.
If you find it too easy, try to switch off auto aim. This was the first thing i've done in GTA4 (i was kinda surprised to see it's possible), and it made the game so much more rewarding.
I use manual aim online aswell. Even if the host is allowing auto aim (and everybody will use it of course). It's just much more satisfying to actually aim and hit, rather than press a button to lock on to your enemy, and then simply hold down the fire button until your clip is empty... i mean, come on, are we in Kindergarten?
Nevertheless, even with manual aim, the game is quite easy, yep!