I've only played with them at the Apple Store, but the iPhone4 silence switch seemed a lot better than the ones on my original 2G and current 3GS. My 2G switch was so flimsy that it would toggle on and off in my pocket from idle friction, while the 3GS one was stiff to the point where I needed both hands to reliably move it.
I do like the feel and design of the 3G model better (much easier to tell if it's silenced in your pocket), but on multiple occasions, the switch on my 3GS has switched from silenced to not. The new one seems a lot less prone to that.
