Someone encouraged me to sign up for one; but I declined.
I didn't have $25 US to throw away.
I'd rather put my money in a jar and spend it later; than to do that.
I'm at the stage where I seem to be more-or-less maintaining (a pound one side or the other), and I'm not going to try and commit to a 4% weight loss, on the hopes I'll get someone else's money (or a share of the pot if more than I reach the 4%).
But, to each their own - if that's something someone else wants to do - I say go for it