I've had a full body cover on my original iPhone (2G) and couldn't sing it's praises enough. 3 years in and my phone looks as good as new. It does dull over time, and it does slightly diffuse the screen (but all screen covers do this).
It has a lifetime warranty so I get it replaced about once a year and when the cover is peeled off the phone looks brand new still. Should help my resale value later on.
I'm getting a cover for my iphone 4, but probably won't put one on the front this time, just the back and sides. The new screen is too nice to cover up. Eventually I probably will put a screen cover on, but for the first few weeks I want all the oohs and aahhhhs a retina display can provide.
/edit Should add I have the full body coverage on my iPad as well and am very happy with it.