Dying internal HD on my 24 inch late 2008 iMac. Decided to replace instead of paying for a new iMac!
The whole process took about 30 minutes...the suction cups made removing the glass a cinch. I followed a guide on the 'everythingnet' with a few alterations...I didn't remove the top cable attachment for the LCD screen and just accessed the HD screws from the top of the case instead...a little fiddly, but I got the job done.
Make sure you have a complete set of Torx screwdrivers!