Binos
Mac or PC, you're in the same boat, given a like-for-like warrany. You're almost always going to be stuffed if you get an all-in-one and the monitor goes after warranty expiry, unfortunately. On the brighter side, I'm on my 3rd all-in-one and it's never happened to me (touch wood!).
As for user-replaceble stuff, no problem. I installed a new DVDR/W drive in my G4 'lamp' iMac, prior to donating it to my other half when i got the new one (shiny G5 2GHz 20"). Incidentally, she's turned into most evangelical Macista in just two months and hardly touches her Tosh laptop anymore! Nothing to do with me, you understand...
I also put an extra 1Gb RAM in both units. I'm no hardware techie believe me, but all ops were straightforward using standard parts (be careful to check for driver avaiability though, if replacing an optical drive).