And, unlike a PC, a Mac will continue to NOT be obsolete for at least 5 years. By that time the OS and hardware need upgrading and it is cheaper to just get a new computer.
Respectfully though I am and always have been a mac person - even in the early/mid 90s when things looked bleak........
But at least 5 years before obsolete? Ha!! You obviously didn't buy one of the G4s before Apple went Intel - the G4s will be hurtin' with the new Leopard operating system. I got 3-1/2 yrs out of my G4 and I feel lucky I did, with the PowerPC chip and the 1.25GHZ speed it slowed to a painful drip with my install of the new CS3. I could upgrade a lot of its parts but why as Apple will phase out the PowerPCs anyway.
If Apple doesn't change "chips" again, etc etc maybe a good life-span for these new macs but 5 years???? I wouldn't bet on it, unfortunately........ Still my G4 still has a life-span just for my husband who doesn't process lots of photos, video etc - but not for me anymore.
I think the new iMacs are kinda neat and spiffy. Not my cup of tea at all - I played w/them in the store. I hate the glossy screen for myself but others think it is really neat. Not a bad price point either. The computer is not marketed towards me but those that are its target audience seem to love it........, it looks like a big iPhone - granted metal instead of black though!
I just got a new MacBook Pro and it is really something else!!!! But at max 4GB of memory capacity I doubt I'll even get 4 years, much less 5+........ I could have gone with a Mac Pro and gotten some more flexibility in upgrading and longer computer life but that didn't suit my needs and it was a lot more expensive, although worth it for the additional power features. Still if Apple changes chip manufacturers again even the Mac Pro will become obsolete............ (of course I don't think that will happen but in theory it could - anything can and will happen in the computer world....). Boy everything 5 years from now will probably all be touch screen like the iPhone - scary thought!!
I've just resigned myself to the fact that I'll buy a computer and in 3 to 4 years (at best probably....) it will become too slow to deal with the ever growing digital camera/video requirements so then I'll just let someone I like "inherit" it for basic computing and fun..... (what are husbands for anyway......) 
I say go Apple and go new iMacs, not not everything is totally great or to my liking but overall - wow....!!!!! - and they said in the 90s that Apple was dead!
Best, Carol