After previously upgrading my laptop with an SSD, (2008 Macbook unibody), I realized how much speed one can get from such an upgrade, and I wanted to do the same for my iMac. The iFixit kit offered me the ability to add an SSD while retaining also the original HDD.
The iFixit instructions list this project as "difficult" - in fact, the difficulty is 5 out of 5. Lots of screws to take out (which I organized in shot glasses so as not to confuse them during the reassembly) and lots and lots of little cables. What made the project so difficult was mainly the fact that there is a lot of sensitive stuff in a very small space. However, the instructions (with one or two small caveats) were very good. I completed the hardware work in about 2 to 3 hours, and then spent some more time installing Lion on the new SSD. (And I am very experienced with this kind of thing.)
VERY IMPORTANT STEP MISSING FROM THE GUIDE: AFTER UNPLUGGING THE MACHINE BUT BEFORE GENERAL DISASSEMBLY, REMOVE ALL RAM CHIPS FROM THE BOTTOM!!! If you don't, you will be struggling with the mainboard much more than necessary, and you may physically break either the RAM chips and/or the connectors holding them to the mainboard. Fortunately, I didn't break anything in the end, but I could have saved myself about half an hour of frustration, as well as much worry and discomfort, had I taken this simple step.
I would also add one more thing. There is a header on the mainboard that looks as though it needs a cable plugged into it, but actually does not. (Probably used for testing or something.) Make a note of this unused connector before your start, so that you won't freak out during reassembly when you can't find the cable that you think goes there.
FInally, this part is in the instructions but I didn't do this; I recommend that, before dislodging the mainboard from its position, you tape down all the loose cables and connectors to their places... after moving the mainboard, those cables have a tendency to wander!!