I've had a 2G, a 3G, and a 5G.
I actually liked the 3G buttons, but only with a proper case (read: iSkin) to provide a bevel around it. When bare the buttons are far too sensitive and get accidently pressed constantly.
USB sucks compared to FireWire, but mostly only for older machines (I don't mean USB 1.1, I mean the massive CPU overhead required).