I agree wholeheartedly. In addition to the social draw to go ("I don't want to miss a class, I have a belt test coming up, my partners need me," etc.), a good school will also engage you mentally as well. Discussion of the theory behind your art and moves (Which is better? Punching faster, or stronger muscles?), etc. appeals to the geek in me.
If you have a kid, enroll them too (if they are old enough), and make it a family activity.
Just be wary of the school. If it's a lousy environment - if you're not challenged, if the instructors are jerks, if you feel like you're being patronized for your money - move on. A good school challenges you (and occasionally you fail), to teach you how to overcome adversity, from inside or without. If you get a belt without trying for it, it wasn't earned, and you're being ripped off. (Chances are, you're not getting into better shape, either.)