My Mac was slowing down, having errors connecting to the Internet, and Parallels wouldn't run 3DS MAX any more. I figured a computer enema was due, but decided to replace the 1TB disk with a 256GB SSD to speed things up.
I followed the iFixit repair instructions and it went awesome! My only hang-up (well, I have plenty) was the SSD's included 3.5" bracket was too short. Fortunately I had a nice bracket from SilverStone. Phew! Now I'm seeing crazy fast boot-times and a transfer rate of ~500MB/s Awesome!
Make sure you have a good SSD to 3.5" bracket with mounting hardware. And keep a can of compressed air to blow out the case's interior, and to blast off the iMac's LCD screen and glass before you reassemble.