I'm not familiar with them. You might try asking on the Community Board; I've seen various discussions about healthcare, insurance, etc. there from time to time.
I have several friends who use these as an alternative to insurance. We will do it also at some point as our insurance keeps getting higher. I have researched them quite a bit. Is there a certain one you are interested in?
They do meet the ACA guidelines and you are not subjected to any penalty.