Rule of thumb: wait for the first service pack to be released before adopting a new OS. That may be more applicable for upgrades rather than a new purchase though.

However, 7 is supposed to be streamlined with fewer bells and whistles, so it may not have the teething problems other releases have. I've been running Vista on 3 computers for nearly two years and it's never frozen or crashed. One of those was pre-installed; one was "Vista Compatible" that I upgraded and one is about 5 years old, but still works fine with Vista. Clean installs on all machines with updated Vista drivers, though, no "auto-upgrades" onto the XP image.

If you can get a great deal on a Vista machine discounted due to the OS, I'd strongly consider that. That's how I got my XP-loaded laptop when Vista was released. Saved a few hundred bucks.