I've bought a few, from Gumtree, and from a second hand shop, as well as from friends. On the whole, it was painless, simply getting a geeky buddy to reload the latest operating system. The processor speed seems to be important, restricting the 'level' of OS (X10.3.9 or X10.4.1) but unless you're running photoshop, and other heavy apps, it seems OK. Like most Mac users, I simply use it, rather than use a PC as a reason to study how to problem solve IT headaches (what is the fuss about those virus thingies?)
It's possible to fire-wire the new one to the old one, and 'clone' the new mac, so the settings & files are already there. My 2003 G4 powerbook lost it's DVD burner when I came off my bike, but a remote CDburner from Micro Anvika works fine.
Apple itself has an online refurb store, they don't have lots and lots of things on there but I would think it'd be a good place to buy from (although it might be more expensive than buying from a person.
I live locally ie SE1 and have an i mac for sale, it's in very good condition as I have now got a lap top, I no longer need it, but it's a great lookin machine ! If your interested let me know, it's not expensive !