Use MicroB mostly. Opera sometimes, because it has the convenience of portrait keyboard, but once the currently somewhat-not-progressing universal portrait keyboard for the N900 gets developed into something that works without bugs, I don't expect to see myself using anything but MicroB. For the time being, I'm just not a fan of Firefox (and yes, I tried 1.1). It's got some neat ideas but I'm significantly more in love with MicroB and how it does everything.

The only thing that will make me switch is if I still have the N900 long enough that MicroB becomes so outdated that it's useless.