I had a full-body invisible shield on my 3G for about a month, but tore it off in the end. There were two main reasons ...
Firstly, the front piece wraps over the top and bottom of the phone, crossing from the glass to the chrome bezel - creating a tiny plastic-covered void at the top and bottom of the phone, which you can't possibly squeeze the air out of ... looks ugly!
Secondly, the curved back of the 3G just doesn't work well with a skin. The corner cut-outs required to make a flat skin fit around a curved body don't look so good under close examination, and after a couple of weeks turn into real dirt traps.
(I should add, I've had an invisible shield attached to my ipod touch for a year, and been very happy with it - the flatter shape of the touch is *much* better suited to an invisible shield!)
After I took the original invisible shield off, I decided to use a front screen protector, and just leave the back naked. This time, I got a front skin from Best Skins Ever. The people who say they're made of exactly the same stuff as IS are absolutely right ... I can't tell the difference between the Best Skins Ever cover, and the devices I own that have in Invisible Shield on.... plus, the BSE skin was *much* better cut than the invisible shield .... the invisible shield had a small (around 1mm, maybe slightly smaller) gap around the home button and receiver holes, which was very obvious once in position. The BSE skin is *very* precisely cut - the lines fall into exactly the right place; you really can't tell there's a skin on there unless you catch the light from the right angle.
Plus, since letting the back of my phone go naked, I've noticed how much *nicer* it feels in my hand - much better than when I had an IS on there; the backplate material has a really good tactile feel. So, consider this one more vote for using a screen protector and naked backplate from me!