depends doesnt it really .... can you use it if so yes if not then no .... Buying it for seo link backs isnt really worth it as your be linking to your own domains and ppl wont be really bitting your hand off to pay to put a link on a pg3 site so it comes back to can you use it if so yes if not then no
I think if it's directory related, and has solid backlinks then you can' go past spending the $7 and regging it. If it's just a random PR3 - then maybe not though, depends on how strong the backlinks are, and how you plan to develop it.
i have heard from many that expired domains loose their PR but its worth a try if you are going to promote it.
I bought an exprired domain a month ago week. I back ordered it 7 days before expiring as it was number 7 in google rankings with a keyword i get traffic from. Two days before expiry it was gone, removed from google index. Don't know If I will have to go through sandbox again. I think you get the picture though!