Thanks.
I might try pin change interrupts, as well. Doesn't seem that difficult, and I have two encoders with two channels each, so it'd give me better resolution.
I should say, though, that this is all for a slow, wheeled robot. I could easily do the encoders in loop() with event checkers (probably one change/few ms), but I'm trying to experiment as much as possible. Now just to find the time...