Hi bassturtle, my experience with the RNC is as follows: to get the sweet compression for fingerstyle and also to make it sound acceptable for fast slapping, one needs to shorten the decay time ("release time") down to a reasonable level. However, doing so creates an unpleasant distortion that is most noticeable at low frequencies. When the decay time is up high, the fast runs become blurred by the compression, and you don't get the desired definition in the attack of the individual notes (because the compressors stays on in between). So one finds a happy medium if possible, a compromise between the two extremes. On the RNC, this compromise tends to be on the side with more distortion. I wouldn't say the compressor is unusable that way, just that it's noticeable, so if your style involves a clean sound, you'll probably hear it. On the other hand, if you're using lots of effects or if you like to dial up some crunch on your amp or pedals, the RNC will probably work fine. It works great on anything that doesn't involve extremely low frequencies, like guitar, voice, and B3 (not including footpedals). Since I don't own a 4 string bass that I compress with, I can't really comment on how it works in that context. On my 5's, I can hear the distortion on the E string.
I'd probably choose a DBX in the under-200 range, probably some version of the 165 (A,X, or XT). They're industry standards (more or less), and in my experience they're very reliable. They're also very easy to use, not too many knobs, and the ones with the LED meters are kind of cool looking on stage. They do change your sound a little (as do most compressors, the RNC being one of the more transparent ones), but it's a smooth change, and it's easy to EQ around. IMO they're a good value in that price range.