i just got my extended battery and backside. i am brand new to the incredible, got this one as an upgrade/refurb after my many moto droid fails, so it didn't come with the back. the verizon tech sent me the extended battery and backside for free (thank you, mr. verizon tech guy). since i don't know any different, maybe my opinion should be down-weighted, but i honestly don't think it's a problem. i can't put the silicone case i bought on it now, but at least my camera doesn't hit the table when i set it down, so i don't feel so bad not having a case. i just juiced it up for the first time, so i'll have to report back with how well the battery does.
the other thing i really like about the backside? it cradles in my palm really nicely. to me, that's a plus. anyhow, i'll report back on how the battery does. i've also got a regular battery i had previously ordered, but no standard backside for it, so i have to decide whether to just take that one back or get verizon to send me out the standard back, as well. i'll live with this for a few days before deciding.
btw...holy crap, but this is a crazy cool phone. coming from a moto droid, this thing is like a feather light powerhouse, even with the extended battery in there. >>wow<<