I used to have my trainer friend do it once a month, but he's no longer at my gym, so I haven't had it done in quite some time. He did calipers (which are only suggested if the person do the measurements really knows what they're doing).

I've done both calipers and impedance. I joined the Y after Thanksgiving and when you sign up, they include 3 sessions and measure body fat using both methods. One said I had 28% and the other said 29%. At the same time, I joined a weight loss challenge led by a different trainer who weighed us on an impedance scale every week. Hers said I started at 35%. I go back to get weighed with hers most weeks and I'm now down to 23% according to that one. In February, I had the YMCA trainer check me again out of curiosity and the calipers and scale were pretty close again, saying 21% and 22% (at the time, the other trainer's scale was still saying 28%). In another couple weeks, once 2 months have gone back, I'm planning to get checked at the Y again. I trust the Y to be decent with calipers since they use it constantly, but I don't know why the two places give such different numbers. So long as my body fat keeps dropping, I'm okay with it, though!