Upload & Sell: On
I have two sets of the yongnuo RF-602, and they work well for speed lights and studio strobes. I can trigger lights from across the house through walls (not sure why i would ever need to do that for a photo, but its fun!)
I have a colleague that got 2 sets of the RF-603, and I was not impressed. Everything about the 603 is cheaper, and he had issues with pins receding, poor connections causing reliability issues, and just general frustration.
The 602 uses a proprietary threaded connector on the trigger, which is cheap, but makes a solid connection. The 603 uses the cheapest PC connection with the cheapest PC cable they could find, and we were constantly struggling with bad connections that could not be isolated to the cable or the triggers. I suspect if one were to buy decent PC cables with the threaded connectors, they would be much better, but then you start to negate the idea of a cheap trigger system by spending nearly as much on cables as the triggers!
Bottom line for me is RF-602 is great out of the box, but RF-603 not so much. I haven't looked lately, but I believe the 602 is no longer available. If I were shopping today, I would re-buy the 602s, but would not buy the 603s.