Out of the 400+ transactions I've had problems 3 times.
One was when the guy took a month and a half to send the package...after numerous e-mails and a couple phone calls, I finally found out it was a somewhat irresponsable teenager, and he finally sent the package, after I talked with his dad! (that was some slot car stuff )

One deal, I just plain got stiffed on...the seller just disappeared. She was selling bundled "lots" of old Air Classics magazines...(she stiffed about 6 buyers at that same time )

3rd time was when a drop shipper promised product when it wasn't in stock...after waiting almost 2 months, and after he said he never received payment (I had my cancelled check in my hand while I was talking to him on the phone ) he refunded my money.

I also buy stuff occasionally here on RCU...but after a M.O. got lost in the mail, I decided to open a seperate checking account just for Pay Pal use.
Knock on wood, no problems to speak of since.
BTW, it takes 60 days to get your money back from the USPS if they lose your M.O.