My two penneth for what it's worth...
I went to the USA last January as part of a modular JAA fATPL course. I entered via Atlanta on the Visa waiver permit. On arrival I was asked by the nice chap at immigration what the purpose of my visit was, and how long I was going to stay. When I told him that I would be visiting for ten weeks, he raised one eyebrow. When I explained that I would be doing some flying, he raised the other one. He ummed and arred for what seemed like an eternity and then gave me a big orange folder and told me to wait for someone else to come and talk to me. After about twenty minutes, I was ushered into an office where three immigration officials were waiting to grill me. I answered all of their questions truthfully, gave them the name and address of the flying school where I would be training. I explained to them that I would be doing my FAA PPL and nothing more. It was non-vocational and purely for recreation. They did let me into the country, although the also told me that I should have had an M1 visa.
I can understand their caution, since I could have appeared to them as one of Osama's buddies, being blond haired, blue eyed, having spent sixteen years working for a major airline and also being married to an American!
My experience? Tell the truth, don't hide anything, be polite and friendly and they will be ok. The flight school where I trained was very small. Even smaller than I had imagined, with only three Cessna 152's and two instructors. That said, the training I received was absolutely first class. It must have been as I somehow managed to get 100% for my PPL written exam and a first time pass for the flight test.
Just for the record, I paid all the fees upfront before leaving the UK and the training wasn't in Florida!
There are good schools out there and it is possible to get in without an M1 visa.