Apple Store was $600 and WRONG - I fixed it for $99!
My daughter's MacBook Pro "lost" its battery - wouldn't hold a charge, wouldn't run unless it was plugged in. I took it to the Apple Store and they refused the repair, claiming it had been wet. They based that on a small area of corrosion right next to the battery connector - we don't remember it ever getting wet but they wouldn't fix it for less than $600, claiming it was a logic board problem.
She used it as a desktop for a while but ultimately it kept coming unplugged while she was working and it became unusable. I decided to ask on iFixit and see if the community thought trying a battery was worth the effort before I threw it onto the scrap pile and bought her a new machine.
I ordered a battery kit and received it a couple days later. I was delighted to see that it came with all the tools I needed, included the adhesive remover to get out the glued-in battery pack. It took me nearly two hours to get it out, get it cleaned up, and get the new battery pack glued in. When I put the last screw in and turned it on, I was thrilled to hear the startup sound - with no cord required!
The instructions were nearly perfect. There was one step where there's a small potential pitfall, but I've already commented on the instructions with that gotcha.
My biggest piece of advice would be that it absolutely takes as long as it looks like it will - but it's so worth it to fix a perfectly good computer instead of being forced to throw it away over a battery.