In reply to plexiglass_nick:
Guide mode is brilliant, especially if the last pitch took it out of you, since it locks off the mode when you take in, so is less effort, especially if the second needs a tight rope or falls off. It also gives you a bit of opportunity to change gloves/eat/drink/take a photo, or if the second is climbing fast, it's easier to take in fast.
But I don't use it for every belay, only where I can get an unquestionably bombproof anchor in a suitable position just above my stance (if the angle isn't right, it can be hard to pull the rope through). If you're swinging leads, you also have to secure your partner, take them off belay, and the reattach the ATC/Reverso to your harness in 'regular' mode.
So guide mode isn't always possible, or worth the extra effort; but when it is, it's really worth it.