Hi.
It is usually a pretty nice community all-round, yes. Part of the fun, really. I mean the cartoon is nice and all, but the community, and especially the creative parts of it, is really something to behold at times. The speed of which a new character gets turned into a fan-plushie is just... crazy. Like it's measured in hours after the episode airs. Myself, I've taken up drawing due to the show. Wanted to see if I could draw one of the ponies. So, I got the show to thank for that. My avatar is self-made, for instance.
And I think a lot of us will happily admit that "adult male fans of My Little Pony" does sound kind of amusing, but that's really how it is. It's not ironic, it's just a cute little show about surprisingly well-rounded characters doing day-to-day things with each other and/or saving the world. Sometimes both at the same time. They just happen to be brightly colored cartoon ponies.
I think the sincerity in the community you've spotted comes from the show itself. It just dares to be what it is, and doesn't go for the obvious wink-wink, nudge-nudge bits of adult humor that so many cartoons do these days. Not that it goes completely without references that would go unnoticed by children, but it never really revels in them. It's just what it is. My Little Pony, but good.
I suppose all I can say is, if you're curious, give the show a try. Or don't. It's up to you. If you want episode recommendations, we can help with that, I'm sure. But just going from the beginning is fine, too. That's how a lot of us started.
Nice of you to drop by. I haven't actually seen Bronies, but from what I hear, it's a fairly decent documentary.