This is a really subjective question and you will get many different answers.
I think, if you are a winning player, even at the micros, normal freerolls are a waste of time. Sure there are special events with $1,000 or even $10,000 prize pools - these are probably worth playing. But the $50 tournaments with thousands of entrants are a huge time soaker as you can play for hours, get fairly deep and still only win $0.25.
That said, there are still a few worthwhile freerolls out there (most of which are available through Cardschat) where the fields are fairly small and so prizes aren't diluted as much and you actually have a realistic chance of winning a few buy-ins to supplement your bankroll.