I've been using RMS 4 and I've been on the beta for RMS 18 for a while. I've enjoyed using it very much and have found that it is quite easy to get great looking renders fairly quickly out of the box. For anyone making the move from Mental Ray, as I was, it's quite a jump but one well worth making. There is a lot of information and knowledge to gain from making the switch and although no one becomes a master at using RM in a short period of time it is a transition that can be gradually made over time with great success.
It's definitely a commitment so be prepared to work and to learn.