Scariest upgrade I've ever done...also, close to the easiest.
I bought a Mac Mini with a regular HD a few years ago, planning to use it as a backup/media storage center. After growing disillusioned with my MBP from a year before, I decided to make the switch from MBP to iPad, and elevate the Mini to my main desktop computer.
But that HD had to go. I have a 4x 3.5" enclosure for storage, so my main concern for the Mini was that I needed as much space as my MBP, but not much more.
After browsing iFixIt to see how complicated the upgrade would be (lots of steps, but nothing crazy like glass removal), the 250 gig drive with tools seemed the perfect solution.
Oh man...amazing. I had all the tools ready to go (not something I typically have on hand) and didn't need to ask around at hardware stores for special tools. The guide was perfect, I never once questioned whether I messed up or pulled the wrong thing, which can be scary to do with a system that's HELLA SMALL, MAN.
The only trouble I had was reconnecting the %#*@ antenna wire, as the small piece did not jive well with my tired fingers, but I did get it in after a brief break.
Take a piece of paper with scotch tape, and right every step where you remove screws down, and tape or lay those screws to/on the paper so it's clear with step requires which screw.
That solved my absolute biggest worry, losing a hard to replace part.