The repair rates on all in ones tend to be higher because of the why they are built. You can't use most generic parts such as drives and boards so you're back to the maker for repair all too often.
As to longevity I find the owners play a big part in that.
I have a (reliable) HP desktop Computer, Win7. Very satisfied, but I keep looking at the All-in-ones as a replacement - next year.
I also read bad things about All-in-ones, that they don't last as long as desktops.
Just wondering, what everybody thinks about Allinones.