The first battery I received was faulty - it kept suddenly cutting out while in use - but iFixit were brilliant in sending a replacement immediately.
Hint: if after changing the battery your Macbook doesn't "bong" when it powers on (and you'd like to to!) then you can fix this by resetting the PRAM. See https://discussions.apple.com/docs/DOC-3...
Don't forget the Torx T5!
I already had a Torx set, so only bought the P5 driver, but turns out my set only went down to T6 :-(
I followed this guide to swap the HD for an SSD, and the main problem was finding a suitable way to mount the SSD. You need a 3.5"-2.5" bracket which has the outer mounting points of a 3.5" drive and accepts the same large-threaded screws as a 3.5" drive. Most are either shorter than a 3.5" drive and/or are threaded for smaller screws.
In the end I found this plastic bracket:
But when the drive was mounted properly in the middle of the bracket, the SATA data cable did not reach to it. So I cut away one of the plastic fins, and lashed the drive to the near side of the bracket with one screw and a cable tie right around it. It does the job surprisingly well - I don't think it will shake loose, and nobody can see it anyway :-)