Apple's new iMacs are a stunning piece of design, that are just 5mm thick at the tapered edge. See: How to install extra RAM memory in the new iMac and save on Apple's high prices
But a recent report by iFixIt.com showed that the 21.5in iMac model was practically impossible to upgrade. Especially the memory which is buried beneath the logic board, requiring you to "take apart most of the iMac to gain access". The iMac display is also fixed using glue, rather than the magnets of the earlier board.
iFixIt.com downgraded the 21.5in iMac to 3 out of 10 for repairability (down from 7 out of 10 for last years model).
It turns out, however, that the 27in iMac is much easier to upgrade. Especially the memory which has a pop-out slot. Just press a small button above the power socket and a plastec cover comes off the reveal access to the memory. Once you have the cover removed you can insert the new RAM, replace the cover, and carry on with your upgraded computer.