Going on my 4th day with mine. A definite and noticible improvement. It's not like it makes it a new phone or anything, but I was about to purchase a spare battery anyway and (no offense to others), I think the extended battery looks horrible and takes away from the phones ability to slip into pockets and etc.
I am a moderate user. Calls, web surfing, face booking. Lots of texting and email with some forums and games here and there. Occasional youtube stuff. I'll charge the phone over night and start using it around 6am. So far, I'll go 12 hours and still have 60% or more left at the end of the day. I can bring it down ito the 50s if I am using it into the evening. Again, not a power user, but I am doing something on the phone at least every hour and often times more.
I think the phone/battery still needs to calbrate itself though. For example, I've been up for an hour and a half and that's when I unplugged the phone. The phone has seen about 15 mins of solid use. checking email, checking weather and stuff while making breakfast for my kids. It still shows 100% Then late yesterday, I had a drop of like 10% in an unusually short period of time.
I have not noticed any temp issues.
Thanks to the OP. I was going to buy another stock sized battery anyway. Even at the $29.99 I ended up payng for this it was well worth it to me.
UPDATE: Not that anyone cares, but I am stock, unrooted on the GB OTA.
just my $0.02