Everyone's right that surplus masks are worthless. But having a good gas mask can never hurt. Some tips:
(1) Make sure it and the filters you buy are new, sealed air-tight (esp. the filters), and have unexpired shelf lives.
(2) The mask should be NIOSH-certified (e.g., the one at the link above is).
(3) Make sure the mask fits well. When it's on, put your hand over the filter hole and try to breath. The suction from your attempt to inhale, combined with the mask's seal around your face, should compress the mask. If that doesn't happen, or if you can hear or feel air being sucked into the mask, then it isn't airtight.
(4) Be aware that if you have a beard, the mask might not seal as well.
(5) If you wear glasses, they'll have to be removed before you put on the mask. A lot of masks have add-on lens inserts that you might have to buy separately.
(6) Be sure to practice using the mask, even though that will involve using up one or more filters.
BTW, here's the mask I have, and I'm very happy with it:
http://www.botac.com/micsafm95.html