I've had a GP 100 for a couple of years. I'm pretty pleased with it overall--it runs a dozen different effects at once and has lots of cool delay/harmony settings. I use it only for effects. The preamp models are pretty accurate, but they are slightly lacking in dynamics. I prefer tubes anyway. My power amp is a Mesa Boogie 20/20, but I think it would be fine with a solid state power amp.
1) Some of them have a problem with the parameter/number dials. You can turn them forward, but the processor sometimes thinks you are going backward or something. So switching and adjusting things can be a pain (although it isn't complicated). Mine has this problem and it drives me nuts.
2) I think the compressor is terrible. I use a dbx 166a instead.
Other than that, I'd recommend it pretty highly I guess. Happy hunting if you decide to get one.