Like getting a brand new computer!
I was replacing the heat sink material, when in disassembling the computer the plastic connector to the keyboard on the logic board broke. So I had to replace the logic board. Then it turned out I also damaged the keyboard ribbon cable when trying to jam it into the broken zif connector - who designs these things, anyway? And you can't just replace the ribbon cable, it's the whole keyboard or nothing.
ifixit instructions are always fantastic - where they color code the screws and then use the same colored circles on the photos to show screw placement? Brilliant. They also sent me a new spudger and (either with this or the new logic board) a great 3-bit driver with a rotating cap that is *perfect* for almost every screw in there except for some reason the torx.
Repair went fine. I have to say, if the web site didn't say "used" when I bought it, I would swear in court that the topcase they sent me was brand new. The computer looks like new, so basically I can enjoy it as a "new" computer for just a couple of hundred dollars
There's a few screws and/or parts that need removing from the old top case into the new top case that aren't shown in the instructions - be sure to read the comments too.
I rated this "moderate" because honestly you're just removing a ton of screws and manipulating a few connectors out, it just takes patience and perserverence rather than any advanced skills