Currently learning, and I was advised as follows:
"Sit in it, make vroom vroom noises, look around, and ask yourself if you think you would be comfortable/happy spending the next 40+ hours being stressed in that plane"
But in all honesty, the advice was sound, and was to imagine whether I wanted to spend a whole load of time in it, and whether I felt relaxed, claustrophobic, safe etc.
Whilst I am sure its true that you can learn in most things, I was told you are more likely to stick through to the end of the training if you like the aircraft you are sitting in, and less likely if you get up on a dreary day and think "I don't want to spend the next hour or two rattling around in that old rustbucket.
That is why I have ended up in microlights. And whilst Ghengis is right that its not an efficient way to get to a full PPL, that didn't matter much for me day 1. If during the process I get a yearning to change, I will, and if not, then not.
(and no, I am not loaded, and therefore free from making those kind of decisions. I just know what gives me a greater chance of sticking with things when it gets stressful).