Most guys would be better off spending the $60 it costs for a drum dial on a couple of lessons from a drummer who can tune. Have him tune a snare, tom, and bass drum while you watch and listen. Knowing what to listen for is half the battle.
The drum dial gets you close, but I've never heard a drum tuned with one that didn't need lug-to-lug fine-tuning by ear. Since you have to learn to fine-tune by ear anyway, why bother with the DD?
They are sometimes also thrown off by the inevitable imperfections in heads, collars, hoops, out of round shells, bearing edges, etc. One of the most common questions on the forums is "Why does my drum dial read high when the tension rod is practically falling out?" Ordinarily if I take a drum dial to a drum that sounds perfect I get varying readings around the heads--probably due to these imperfections.
It's best use is to easily replicate a previous tuning without having to re-invent it from scratch every time you change heads. I tune to pitches so I can do the same thing, faster, with a $20 pitch pipe.