Forgot to add my own opinion, since I know everyone's just dying to hear it. :-D On the whole I'm rather neutral on "brony." It's an entertaining portmanteau that rolls off the tongue easily, and I do more or less fit the demographic...though with some of the more
enthusiastic representatives out there I'm a little less likely to loudly advertise myself as one of that bunch. I'll continue to enjoy the show as one of my various interests, and I don't mind being called a brony, I just don't really feel a need to wave a flag about it.
Pegasister I don't personally care for though. It's long and cumbesome for one thing. For another, I don't really see any need to subdivide further--there's the target demographic (what was it, girls aged 3-11 I think?), and there are the fans outside that demographic; anything else just feels like it's unnecessarily complicating things. Plus on a more personal note it reminds me too much of a girl I knew in high school, Peggy, who went through a phase of insisting that everyone call her "Pegasus." Just...awkward.
But as Lofty said, labels like these are more about saying what things you like. You can label me a brony, gamer, Trekkie, nerd, geek, stylophile, clotheshorse, metalhead, musician, artist, horseman--I don't care, they all apply to some extent and none define me exclusively. But if you don't like being called a brony/pony/pegasister/whatever else, that's cool too.
Sethisto's incredibly creepy 'Twixie' fixation, anyone?
Which is tamer than his Cynder infatuation was, if you can believe it.