I mean I currently run a 5870 purchased quite a while ago. It is still a very good card 2 years later. That being said from my personal experience when I upgrade it is usually a whole computer. Why waste time buying a 2 year old card when you can get the latest architecture that should out perform this dated card for a few bucks more. Also giving you better eye-candy. I usually upgrade every 2 or 3 years so I like to hit the sweet spot with the best bang for the buck which two years ago was this card by far.
Now this particular 5870 is second hand meaning someone else has already used it correct? If so check the manufacture and find the warranty specs on it. Certain brands carry lifetime warranties or double lifetime warranties that transfer to new owners.
Basically if you can find it at a steal sure go for it. Still a reputable card that will perform pretty decent in most all games medium to high settings. It will probably actually make your CPU the limiter in most newer games. If a 6870 or 6950 or 560TI is not much more in price though I would go with one of those.
If it were my choice I would wait another 2 or 3 months on AMD to put out their new GPU architecture (I am sure Nvidia's will not be far behind). I believe both of these are going to smaller dies which will push current cards prices down. Plus they are supposed to have really high performance gains. I would save my money and wait to see what the new releases have in store for price/performance.