Hmm... perhaps I'm going to make a sweeping judgment here, but nonetheless I base it on (a lot of) personal experience.
I would avoid ANY distortion pedal which is labelled "Metal". I have tried many, such as the aforementioned Boss Metal Zone and its Digitech equivelant, plus a variety of other distortion pedals which are labelled "Metal" and, to my ears, they ALL shared one common denominator; they all sounded like utter garbage. Horrible, tinny, noisey, and absolutely NO chance of being heard on stage or in a studio in a mix.
Best way is to set your amp up to get a "Metal" sound. What amp are you using? In most cases plenty of Gain, Lots of bass and treble and a little mids will get you a decent metal rhythm sound. Don't take out too much middle or you'll never be heard in the mix if you're playing in a band. And use an EQ pedal to add mids for a lead break...
'87
P.S. Other distortion/overdrive pedals may be of use however... just avoid anything labelled "Metal" because it is bound to sound garbage.