The RNC is a fast, peak detector compressor/limiter. I use mine primarily on the way in for recording vocals, acoustic guitar, overheads and bass to add a little bit of polish. Just a bit of fast limiting really makes the difference.
The Really Nice function is great, adding a more forgiving RMS detection style compression character. I've used this when running things back out to it, but am a bit cautious of adding that kind of character during tracking.
My main problem with it is that the attack and release times can be so fast it can be hard to avoid bass heavy instruments distorting. The best workaround for this is to add a HPF to the sidechain - easy really, just a TRS jack and a capacitor. I leave mine in for the most part and it works great.
It's probably the best compressor in its pricerange. If I went back to a desk based studio I'd probably want 3-4 of these before I got anything else.