30 minutes - 1 heure
iMac wouldn't power up; was about 6 years old. The Apple store wouldn't repair it, given that it was >5 years old. Took a chance for $140 that it was the power supply - and it was!
Repair went well, thanks to the on-line video - I wouldn't have tackled this project without the video. Spent maybe an hour to take it apart, and maybe an hour to put it back together.
Its not a difficult repair, but go slow, and pay close attention to the instructions in the video (the instructions mention most, of not all, of the pitfalls). Label all the screws and their location as you take them out - you end up with some odd combinations of sizes and heads, and its not intuitive where they all go. You might have to spend some money to buy the specialty tools if you don't have them (e.g. the suction cups and small torx heads) but for the small amount of money you can save your computer relatively cheaply and quickly.