I've tried a number of them over the years, and could never fully adapt. I have big beefy thumbs and many just don't fit, or fit well enough to be stable. Some literally strangled my thumb, turning it blue after a bit. My thumb also has a pronounced back bend, so the thumb pick surface does not strike at a good angle. Fortunately, I am blessed with pretty good natural nails for my finger picking. Unless I'm playing more than ~90 minutes a day, they grow out faster than they wear away.
Of the thumb picks I've tried, the Fred Kelly Speed Pick came the closest to working for me. Everything else has been way too thick, or too thin, just did not fit, or had some other issue. I have similar problems with finger picks too - Alaska picks are the only ones I've had any success with (after some filing) and then only on an emergency basis while a nail was healing.
http://fredkellypicks.com/product-category/speed/
