Depends on the type of music you are recording. The RNC will not add color near as pleasing as the UAD or Powercore. However in most software apps, plugins are applied after the track is already written to the hard drive. The RNC, when set up as a limiter, would control transients during the recording process, something the cards may not be able to do.
If you use the RNC for the compression color, you will be disappointed up against the coloration of something like the 1176 or LA2A that comes on the cards. This is not its forte.
Protools will allow you to route into a bus first and then back into a track for real-time compression however, there is some latency with this technique. In this case you can actually use the UAD/Powercore in real time. The need for an external compressor is less in this instance.
Sonar, no way, all plugs are after the write to hard drive and if you do not have a limiter or slight compression prior to the soundcard, your written file may have some hotspots. In fact I use the RNC to limit externally and then when I need color I use the 1176, LA2A or Fairchild from a UAD on the recorded tracks.
Another low cost external limiter/compressor is
the DBX 160x which can be had for the same price as the RNC. I find it smoother on vocals with the over easy switch turned on than the RNC.
On the other end of things the RNC has a nice clarity with the super nice button selected, slightly grainy, but sometimes just the right thing.
