Defintely agree with all the above - speaking from my own experience:
M1 schools have to be specially approved by Immigration in the US and therefore have had to prove themselves.
If you speak to the US Embassy and tell them that you will be completing a PPL in the US they will say that you have to have a M1 Visa to allow you to study in the US. This really applies to those students who need to be int he States more than the 3 months allowed on the Visa Waiver.
It is a 50/50 thing with the nervous people preferring to go the proper route and get the M1 (the paperwork is no big deal) and the rest saying that they are just on holiday.