I just (meaning, like an hour ago) replaced the smoked plexi in my cab with a sheet of Graylite 31, and it is night-and-day better. The image is clearer, the light and colors pop out, the surface of the glass itself looks great... just a big improvement all around visually. The game graphics look like they're floating in midair instead of being displayed on a monitor.
Only real problem was finding a local source... the glass shops around Atlanta wouldn't even return my calls for such a small order. I wound up buying it from a glass shop in my old hometown over in Alabama ($37).
HOWEVER... as noted above, glass is something you have to be VERY careful with, and I have a 3" scar on one of my hands to prove it (long story, fortunately not related to arcade games). Like Santoro said, if kids are going to be around/playing your game, you might want to go with plexi, or maybe have a sheet of clear plexi between them and the glass for safety's sake.