I took the stickers off both cars yesterday (4 HOV on the Volt, 3 HOV plus the EV parking sticker on the LEAF). Seven came off quite easily, but one was a Son of a Bitch!
The six that came off easily, the plastic sheeting that made up the top layer of the sticker pretty much separated from the adhesive, but not so much that I was left with a "VOID" imprint on the bumpers. Instead I was left with a white patch that was pretty much everything except the printing (which appeared to be reverse printed on the back of the plastic sheeting). This residue I hit up with some Goof Off, and it came off relatively easily. I followed this with a spot wash of the area with soapy (car wash) water.
The one sticker that was a S.O.A.B was the smaller HOV sticker on the back of the LEAF, and on it the plastic sheeting would not separate from the adhesive at all. Plus the Goof Off had zero effect on the plastic. So I started to scrape it off with a fingernail; then got frustrated with the pace and went to a utility knife blade. The latter mostly worked OK, but I did end up jacking up my paint a tiny bit. I covered those boo-boos with a GB country sticker. If I had it to do over again, I probably would have left any sticker giving me the slightest resistance in place.
Fading around the stickers was about what I had expected - somewhat noticeable, but not to the point that it would be obvious if you didn't know where to look.
2011 Blue Ocean SL with 78,000 miles
2015 pack on 12/30/15
Bridgestone Ecopia 422+ tires
L1 EVSE upgrade
Superbright LED lighting
2013 sun visors
GT-R map lamp lenses
Altima illuminated door switches