I've tested almost all Drum VSTi's out there back when I was on the market and also heard and played with the RDK a bit. In the end me and my band choose Addictive Drums.
To sum it up why:
- Loads faster
- Better sounding IMHO
- On-board usable fx (compression, verb, distortion etc.)
You can buy additional kit pieces, like a Ludwig snare for $10.. that's kinda cool. I begun to sample AD to Kong by the way
The RDK is a drag to set up in my opinion and doesn't sound half as good as AD.