I pronounce it MEG-ahn when it's spelt
Megan - the conventional way. However I kind of like
Maegan, which I would pronounce MAY-gen (though I think this may the American way? Not sure as I'm English living in England.)
I know three Megans, one 18, one 16, one 13/14. It's popularity doesn't really put me off but I think
Megan is a little boring, which is why I prefer
Maegan.