I got used to clipping in at spinning class a few months before getting my first bikeMy trainer is perplexed as to why my feet come off of the platform pedals, and they even do it a little on the Aerodyne (however you spell it) at the gym when he is trying to raise my heart rate during one of our training sessions.
I have decided to purchase the M540 pedals, they are just a little more expensive than the M520, and they are lighter - though seem the same size so it must be the materials. I am, well, nervous about making the switch - but it also makes sense to me. If I am having to fight to keep my feet on the platform pedals (and they tend to slip off at the worse times), then that can lead to a fall just as much as getting used to the SPD pedals...
Thanks everyone!