I've been using GoWearFit (they're both at bodymedia.com now) for exactly one month and I LOVE it. I've lost 11 pounds, which is slightly more than the numbers say I should have lost - but close. And I'm not complaining.
It has been very useful to me to see just how much my daily routine varies. I knew it varied a lot - I just didn't realize how much.
It has also been very useful for me to monitor my sleep. I've found that I feel best if I can get at least 7.5 hours a night. Great at 8. Not so great at 7. 6 is gruesome.
I am sure other more experienced folks will chime in . . . just wanted to give my two cents.