I really like my MPX500, and I think if you are going for a studio processor (for use on everything...vocals/drums/bass/guitar/etc...) Lexicon products outshine all others. However, if you have the money, look at one of the PCM units instead. Those are used in the professional studio domain and you can hear a difference. Either way, stick with Lexicon (over T.C., etc..).
So "silent" when needed and so "punchy" when needed, compared to my other gears (Alesis Quadraverb, BOSS SE70, ...) .
It is a real professional tool at semi-pro price !
You'll be disappointed only if you're looking to reproduce the precise ambience of your nearest cathedral ... due to limited number of editing parameters !
But in most cases, no one is looking to reproduce the exact ambience of a given place...
You can also buy the upgrade chip for the MPX500 which will add compression. Not too many Multi-FX have compression. As much as I like the unit, I prefer my Ensoniq DP Pro's over it, but I do like to have a bit of variety.