My iPhone 3G is in need of a new battery.
I can buy a kit on line for about $18 which is for sure the right price.
Has anyone done this? What was your experience?
I did a Gen 1 iPhone. It was pretty difficult, and very hard to do without messing up the case. It also required a pretty darn tiny soldering iron. I don’t know if the 3G is any better, but I doubt it.
According to the online instructions the 3G is way easier. No soldering.
Well, my only piece of advice is: don’t force anything.
I have not, but my son-in-law replaced the screen on his 3GS. He said it took a bit of effort, but when his wife dropped her phone in the pool, he said he had the battery out in 45 seconds. I guess it gets easier with practice.
Standard mechanic’s rule applies:
Don’t force it, get a bigger hammer
Trust me I don’t plan to force anything
I call shennannigans. There are ten tiny screws to be removed and about as many tiny ribbon cables to be disconnected in order to get at the battery.
Rick - the iPhone’s battery is Apple’s equivalent to a heater core - everything else is attached to it, working outward. 
iFixit has nicely detailed instructions on getting into pretty much all things Apple. iPhone instructions are here. It’s all very small and fiddly, but certainly doable by mere mortals.
Perhaps he just had it disconnected (not removed) quickly - still, the main point is that it was easier/faster the second time.