I've only played Rage, or I think its Rage HD (?) on iPhone4 and its really good. No touchscreen device is ever going to match an actual consoles stick/button experience or graphical ability so pointless comparing the two. Modern Warfare running on iOS is a very good game once you get used to the controls.
Problem for me is that I played Rage HD then transferred it to my iTunes App Library to free up space on the phone for other apps. After attempting to upgrade to iTunes 10.5 (the latest one which I was told was necessary to download and install iOS5, it messed up everything. I can't sync the phone, can't transfer anything to or from the phone to computer, and can't even access iTunes store now. So please Apple, get you act in order because according to the Apple forums, alot of people have the same issues and expect some answers.
iPhone is great when it works but, to anti-quote the late and esteemed Mr Jobs..."It just doesn't work!"