I've stumbled across a few RADAR bills over the last few years and I've kept each one but would not consider myself a radar bill collector. I only kept them because I know others collect them and I've sold a few to coin shops for above face value, and much higher in store credit. I was hoping to sell the few I have left, along with a few other items I don't particularly care to keep any longer. It's good to know that there is a market for them.
I still check each bill I get, but looking for my phone number instead of radars.