Road conversions have slack (comfy) angles, but lower bottom brackets, leading to frequent pedal-strike (where the pedal hits the ground when you turn). I'd not recommend them. Purpose-built fixed gear road bicycles (like the Surly or the IRO) are good choices for general riding. If you like a really responsive, twitchy ride, and are maybe thinking of taking it to the track, consider the Bianchi Pista, or getting a track frame and building it up yourself.

I don't know much about the langster (specialized), but check the head-tube and seat-tube angles. ~75 degrees is more "trackish" and ~70 degrees is more "roadish".

I DO think that conversions of old horizontal-dropout MTBs are awesome bikes though, as they have the high BB's, and general "not like every other bloody fixie I see everywhere" value. But you need a 135 mm hub for these.