I have had the rx-65 for years and have not had any issues.
I was intrigued by the refurbished 8500s and the S7 horn from what I read on this board so I decided to get one for my other car/for fun to play with and compare.
1) when trying to run them together the 8500 picks up a K false from the rx65. at least when its mounted lower and in front of it. Side by side I didn't notice as much. I only ran then together the first day just to make sure the 8500 wasn't completely defective.
2) I notice the reported 'better off axis' detection of the S7 by my daily RideAid false I pass. The 8500 is picking up a much stronger signal. They both pick it up without fail, but the ramp up and overall strength detected is much higher on the 8500.
3) I got a nice K-band save with the 8500 in the fog, and going over a hill. Probably a half mile overall detection and way before line of sight over the hill. The over the hill detection was impressive and I was not running the RX at the same time because I was worried about interference.
Overall, I really feel comfortable using them interchangeably. I wouldn't feel any more or less secure with one than the other at this point.
And if I did not already have either one, I would definitely go with the 8500 without question for its combo of price, warranty, and capability.