I started a similar thread a while ago.
http://aussiepinball.com/index.php?topic=8408.0

I think a good mod is one that is tasteful, not tacky and can be removed so the machine can be easily returned back to it's former glory to suit the next owner.

eg mirror blades look great in some machines with black mirrors being my favourite as they are still close to what a cabinet looks like but with a little bit of bling and can be removed, as opposed to those new cabinet side decals instead of mirror blades are such a permanent mod and would be a pain in the a##e to remove and fix the cabinet.

wotto loves toppers, so here is a mod for him  , but not to my liking