I always carry one, but only use it for two things:

1. Like Lori, determining for dead sure which trail to take at a junction, when there are no signs (or it looks like someone might have messed with them, just for fun.) It's also useful if there's a side trail that's used enough to appear to be the "official" trail.

2. I use it on the infrequent occasions that I want to go off-trail. Usually, it's just to go a short distance (less than a mile) to jump from one trail to another; the basic technique is to walk a bearing. Elaboration occurs when in the woods, when you choose a distinctive tree to walk to, then find the next one along the bearing, and repeat until you're where you want to be. Occasionally, you'll have to offset by walking 90 degree offsets and then returning to the original bearing.)

I don't do either of these much, but I always carry the compass anyhow - it's an ounce or two of weight for an item I don't want often, but when I do want one, I want it badly.