A good one is awesome (older, US made version with LM308 chip). I haven't tried the new ones but I hear they don't sound as good. FWIW, I used one with the drive set low, the filter on the warm side, and the output up to goose an already overdrive amp. With the gain cranked they become more of a fuzz pedal than a distortion to me.
I use a RAT. Its killer. I dont really use it for distortion, but more for the tone I get. Set the gain on 1/4, filter on 1/2, volume like 3/4. It boosts highs. Leads sound really nice with this pedal. I couldnt live w/out my RAT!~!8O
edit: I use it in front of my high gain Marshall tsl100 full stack. With the gain on my amp at around 8
Yep, definitely a unique pedal. I have a new Proco Solo which is a new version of the Rat with a scoop knob and 3 flavors of asymmetrical clipping. I'm liking it so far, but I don't use it as my main distortion, just a boost.