A few months ago I picked up a new M4 RX-65 and I've had some time to get familiar with it. I also used an S7 955 for about two years.
I've been keeping the 955 in my Miata and I haven't really been using them (the car and the detector) very much until it got warmer recently. Just this morning I was on the highway approaching a construction zone. I got three full power IO warnings (the one where it goes BEEBEEBEEBEEBEEBEEBEEEEP beep... beep... beep), the most distant one being at least 5/16 mile away from the source, a handheld gun of all things (didn't identify it, unfortunately).
Now, this made me sad because I've had handhelds pointed directly at my car before at very short range and the RX-65 struggled to give the IO warning. Yeah, it beeped, but not with the "you're screwed" sound.
I also noticed that the RX-65 alerts more erratically to sources to the rear and to the sides. The signal level is lower and sometimes it will cut in and out on a source the 955 would be continuously alerting to. I understand it was designed that way, but it actually makes it hard to mute some of the falses I drive by because it will beep and then quit and if I hit the volume/mute button it will just raise the volume, but then it beeps again 3 seconds later. It also doesn't really inspire confidence because I need all the detection I can get for certain situations, like low powered guns being aimed in the same direction as traffic where the only detection would be from reflections.
I guess I just sort of expected more from a newer detector.