Eligability for the free upgrade depends upon the manufacturer of your system. In general it is only valid if you bought your system in the second half of 2009 (after the Windows 7 launch date was announced). I also noticed that in the case of Dell they will not accept additional requests for media after 1/31/2010 so if you are going to jump on this do it ASAP (date seems to vary by manufacturer).
With that out of the way I'll state that Windows 7 is a huge improvement over Windows Vista and if you are currently using Vista and are eligable for this upgrade you should take the manufacturer up on it. I've been using Windows 7 since the public Beta starting early last year and have been running the final version since it became available to IT Pros in early July.
However if you aren't eligible for a free upgrade you may want to hold off. The upgrade prices for Windows 7 are a little steep (depending largely on the version you have); so you may not want to spend the cash.
The upgrade process is straight forward (did it on both my laptop and on my parents' desktop), but it does take several hours so plan accordingly.
Brian
"My father was very sure about certain matters pertaining to the universe. To him, all good things - trout as well as eternal salvation - come by grace and grace comes by art and art does not come easy." Norman Maclean, A River Runs Through It