I'm using an iBook G4 (1.33 GHz) since 2005; had to replace the harddrive in 2007, no problems since then. Currently running OS 10.5.8, with no problems.
I use MS Office 2008 for Mac, but can't upgrade to Office 2011 till I get a newer model with an Intel chip. True, there is software I can't use till I get an Intel Mac, but here in the Mid-East and for my current purposes it is adequate.
I anticipate returning to New York in 2011 with some money and then I hope to get an 11 in. MacBook Air for traveling, and perhaps a 17 in. MacBook Pro for home, and an iMac for my wife. Never had a virus (save some I picked up from copy stores and their PCs with a flash drive, but Norton caught them right off. They wouldn't have hurt my machine, but I could have sent them on to others).
Operation is simple, and the machine is rugged. When I buy hiking boots, I buy the best, and it saves in the long run. Ditto with technical mountain gear like jackets; I get the best and it lasts a decade or so.
My wife's currently running a Powerbook (with OS 10.3) from around 2000 (only has a 13 GB HD), but she loves it, and wants to keep it. I have to tell her not to get sentimental over an old machine -- she'll love the new one too!
I suppose I could do with a PC if I had to, but it would not be as pleasurable an activity as working on a Mac. I wish you guys the best who can get by on one, but me, I'm a Mac guy, now and (DV) forever.