I actually take the time to look up vids of just about every song that gets released (at least in genres that I know I enjoy, like prog, new wave, metal and rock), but as with official HMX DLC, I'm simply pretty selective about what music I enjoy, and also what music I enjoy playing in the game. In the average Xbox release week, there'll be about 1-2 RBN songs that I'm interested in, which means that in the average PS3 DLC week, I'll pick up about 1 RBN song. It's not much, but it's all I like.
Again, I'm just as picky about official DLC... I've gone as much as 2 months without buying anything in the past. And the biggest praise I can give is that some of those RBN tracks that I have bought rank extremely high in my 'favorite RB tracks of all time list'... bands like Amberian Dawn, Ayreon and Cynic are bands that I absolutely love and would choose over 99.9% of the HMX-released bands, and songs like The Great Plains and Curse of the Werewolf are incredible as well. I especially hope labels like Nuclear Blast and Season of Mist will continue to support RBN as much as they have, because they have a lot of bands and songs that I'd very much like to purchase.