Experience with sleep consultants for toddlers?

Does anyone have experience, good or bad, using sleep consultants? My wife and I are at our wits end with our 15-month old. Every time we get some progress, our daughter takes two steps back. She’s gone from napping horribly and unreliably to not napping at all, and the nights have become a big problem again.

I’d always scoffed at the idea of a consultant, but at this point I’m willing to try just about anything.