they represent an excellent combination of value and space saving all in one as long as you are willing to accept the following:
1. Besides RAM and Hard Drive space, you aren't going to be upgrading much else in one of these systems.
2. When a component inside the CPU croaks, expect to pay a boatload of money to get it repaired (provided it isn't under warranty)
Thanks to the advancements in USB and Firewire based devices 1 isn't that big an issue anymore as most cards that people are putting into their systems (TV capture, wireless, storage, video capture, missile launchers) have some form of USB equivalent. Problem 2 is a couple years down the road when components start dying (usually the day after their warranty ends). My parents have a fairly recent iMac that bit the bucket outside of warranty and in the end it was more economical to buy a new system than it was to get it repaired.
Personally I stay away from all in one systems. Good luck with your decision.