I have two GTX580's and I've had them for more than a year now. They're great cards, but I wouldn't invest in them at the moment. Sure, ATI has some great offerings at the moment (as The Joker pointed out) but NVIDIA isn't asleep at the wheel either and they're about to launch their brand new cards too. With all due respect to AMD, given past events NVIDIA's offering will, more than likely, trump AMD again. Remember, AMD want to give the "best bang for buck" product. NVIDIA are interested in giving the best bang...period. NVIDIA also holds some perks as well such as PHYSX, which is being used more and more in AAA titles and does make a difference in the gaming experience.
If I were you, I'd hold out to see what NVIDIA has to offer in the near future before you go and blow your money on the current top-of-the-line AMD card. The GTX580 is a great card, but it's old tech. Comparing it with the brand new AMD offering is not fair and essentially a silly move. Wait until NVIDIA also have their new card on the table - THEN compare the two. Just don't be surprised if you end up with the NVIDIA.
Edit: Just to add insult to injury - the mere fact that the GTX580 manages to keep up (and even beat) the new AMD cards is a testament to how good the top-line NVIDIA products are.