It's been hella fun. :D laurie_robey periodically comments on how I'll be sitting there at the computer with a goofy grin on my face thanks to something going on in the feed. It's also a more productive waste of my time than several of my other time-wasters of choice -- by which I mean, I'm getting more enjoyment out of it relative to the time and effort I'm putting into it. I've done more pure RP in two months of Twitterponies, for instance, than I have in three years of LotRO.
On the other hand, it's not without its issues. The TP have a set of pretty straightforward guidelines that boil down to "stay in character and make it like the show," which for some reason just give certain segments of the internet fits, and dramas keep flaring up about it. Somebody will create a character who is "Axe-Murderer Pony" or some such, several people will call them on it, then there'll be arguments, recriminations, accusations of favoritism, and eventually Axe-Murderer Pony will start posting about what a bunch of stuck-up jerks the Twitterponies are for ruining their fun. (Interesting how they never say, "Sorry, Twitterponies, for coming in and trying to ruin YOUR fun.")
Also, Twitter is just not a very friendly roleplaying venue. People wander off from their keyboard in the middle of a scene. Spambots pop up at you. You have to do your best to be friendly and approachable, especially to the five-year-olds who post responses to you like "hi do you like twilight are you a pony i love you." It's a strange hybrid of roleplaying, performance art, and being a costumed mascot at a theme park.
But even with all that, I'm having a great time with it and I've made some cool new friends with it as well, and that's always a plus. As much as I thought I would miss gaming when I gave up being behind the GM screen, I've actually enjoyed the Twitterponies a lot more. If you're on Twitter and you like the ponehs, I recommend you come around and give us a watch. :)