I have
a Digitech RP200. Love it. Quality effects, tons of control, tons of simultanious effects, great distortion, and a fully customizable expression pedal. I wouldn't go with anything without an expression pedal (I know a lotta people suggest
the RP50); the expression pedal is just too useful to pass up. I have yet to find another multi-effects unit in the price range that even remotely compares to the Digitech.
I bought mine used off of ebay. Was in great condition when I got it, and I got it for about 50 less than retail (including shipping). After years of road wear, however, I've some how managed to screw up the electronics so much that the tuner doesn't work properly anymore. I don't know if this is a common flaw in the unit, a result of something I did, or a result of the previous owners actions. Still, I put my gear through hell, so I'd say it's held up quite nicely. I wouldn't really worry about buying a virtually new one used, and I'd do it again if I had to replace it.
Oh, and if you take any of my advice from this, DON'T get a Zoom. The effects are crappy, they'll break on you, they're hard to use, and just crappy all around.