I think MP's are a total crap shoot and vary widely depending on where you live.  In Chicago, I think they're a good place to get ripped off or busted.  A place with a reputation for ripping people off will quickly draw the attention of LE.  In New York they seem to be more prevalent, accepted and consistent.  I don't think you can know what to expect unless you find out from a review or reliable word of mouth.  Reviews on specific girls may be hard to find because they seem to have high turnover.