The Crazy Kong cabinet that I converted to MAME already had smoked plexi, but I'm very glad it did. The bezel is just black posterboard. Games with a black background (like virtually anything before 1984) just kind of "appear" with none of the monitor parts or bezel really visible. It's really sharp.
My Primal Rage cab that I use for JAMMA games has clear glass and you can see every imperfection in both the monitor and the surrounding bezel as well as the crappy paint job that Atari did on the inside of the cabinet (should have all been painted black, but it wasn't).
My Neo-Geo 2-slot dedicated came with smoked. I had my doubts being full-screen full-color graphics, but it really does look cleaner and much more finished with the tinted glass. Doesn't dull the graphics near as much as I expected.
I'm going to find a way to implement smoked plexi or glass into my cocktail project soon. First I have to find affordable glass for the top, though.
Does anyone have a preference between glass and plexi? Is there much of a cost difference?