We have two recently-purchased computers in the household that are eligible for upgrades to Windows 7 from Dell. I was able to register one of them (an Inspiron 537s desktop) for the upgrade but not the other (an Inspiron 11z netbook). Evidently Dell has changed their website so that the 11z is no longer listed as eligible, although at the time we purchased (Sept 3) it definitely was listed as such--in fact, Google still has it indexed with the phrase "eligible for free Windows 7 upgrade!" Interesting! I've e-mailed Dell in an attempt to get a straight answer (and, of course, an upgrade).
I've looked at Windows 7 in the "release candidate" edition and it seemed to work well. Word on the street is that it will run well on slower processors (like the Celeron in our netbook) where Vista tends to be sluggish. Actually, I haven't found Vista to be all that problematic once I've turned off the fancy bells and whistles (such as the Aero interface) that look cool but consume too many resources.
If you're happy with Vista (or for that matter, XP), I wouldn't rush out and spend the $100 plus that an upgrade to Windows 7 will cost you unless you see a compelling reason to do so. The upgrade from Vista should be fairly painless but jumping up from XP will require a "clean install" which means all your documents will need to be moved and all your software reinstalled. If you've got your software organized and your documents backed up (Seagate 1 Terabyte external drives are just $109 at Walmart!) then you're good to go, either way.