I recently acquired a small pile of incomplete sets from eBay. They were listed as a job lot and I won them for a very reasonable price. Non of them have boxes or instructions, but the just helps keep the price down.
They came ready assembled, but a look at Peeron.com showed a few errors, and just what was missing. All in all I think I got a bit of a bargain.
As it turned out, two of the sets - 655 and 689 were complete and there was only one actual broken piece. I think it was worth it for the old rarer pieces included.
I already had a boxed set of 604, but looking at the sets from 1971 I saw that 682 had an identical model, along with other stuff, so I can put my little digger toward that set.
I'm one of those (possibly weird) people who would rather get a sack of mixed Lego and spend weeks sorting through it to see what I could make up than get a load of complete sets knowing that there's no real surprises in store! If I could win one of those auctions on eBay for 150 kgs or 200 kgs of Lego I'd be in heaven, and I'd probably never go back to work!