Hear, hear!
I've never had occasion to use them on my six-strings, as I don't have a problem with bellying on my them, but for 12-strings, they're virtually essential.
I've got Bridge Doctors (the screw-in version, which I prefer -- see below) on all my good 12-strings, which is to say, on all my Taylors (four) and Guilds (eight, with a ninth and 10th on their way!).
I swear by 'em. They not only flatten tops (and that they do VERY well), they can improve intonation, depending on how much tilt there was to the bridge, and the improvement in sound is, in some cases, dramatic. Most of my Taylors sound better and all my Guilds do.
The screw-in version also has another benefit for Guild owners. Guild 12-strings are notorious for bridge lifting, as we know. The screw-in Bridge Doctor serves to secure the bridge to the top of the guitar at the point where there's the most pull, which takes care of minor bridge lifting without a trip to the guitar vet, and they'll prevent future bridge lifting if your guitar hasn't yet developed such symptoms.
For what they do, they're cheap ($30.00, plus or minus, for the screw-in version), and they're easy to install (and they get easier every time).
As far as I'm concerned, the Nobel Prize in Engineering should have gone to Don Kendall, the inventor!
Glenn//.