Quoted from Excalabur:

20mW (Class IIIB) is dangerously bright for a laser that you're going to shine on a pinball table, which is noted for, amongst other things, having a f!@#$ing shiny ball on it. Class IIIB is defined as 'reflections off of shiny things can do eye damage'.
Don't go higher than IIIA, (which I think for 405nm is around 5mW?).
In general Blue (GaN) laser beams will look really dim in comparison to other colours for a given wattage. This is due to the response curves of your eye, but has nothing to do with laser safety: the damage don't care that you can't see the laser. This is why laser pointers are generally not blue: getting them bright enough to see generally makes them unsafe. Furthermore, chinese lasers are renowned for not being made to spec---our lab had several blue and green laser diodes confiscated by Australian customs for being too bright and therefore 'weapons'. You probably don't have access to an optical power meter, so err on the side of 'too dim'.
Don't put your eyes out, folks. Note that the damage you do a) might not be immediately apparent and b) won't hurt except in usual circumstances.
(I'm a physicist that works with GaN laser diodes on a daily basis.)
Post edited by Excalabur : grammar