I have bought PR6 domains on the drop and they were PR0 by the time I got them. Always check quality/quantity of backlinks.
I would have to agree with Mong. Do a backlinks analysis first. I would also check several places first to check PR as well. Check it against your google toolbar, iwebtools, seotools, NP whois and whatever place. I have bought a few domains with PR. Some held their PR some didnt. And if the domain has PR with backlinks. Be sure to get something up asap as to not lose its PR value. I had that happen on 2 domains. And be sure you do a search on the domain name as well. I bought a domain with a PR1 which ended up being and adult chatroom. Didnt know it. Until after buying the domain and receiving emails in inbox from adult website spammers. So I did a search and found out what it was. Pfft. So research, research, research. Than buy, buy, buy
i would only buy an old domain for its type in value, or stream of residual traffic, i ve checked quite a few domains in a niche of mine that had existing PR an found all the links to be spam html postings on other folks website
In my own experience if domains are live they can be a good deal, but if they are dropped you are wasting your money and time.