My experiences are thus. I own 2 DX5's, the first one stopped binding after I dropped it. Totally my fault. It still works with the simulator though so I plug it in and let the kids use it. The other one I use with my foam cub when my kids fly it, and as a trainer box when people want to try flying.
I have friends with the DX6i and the old DX7's, and they fly the same things I do and have never reported a problem. I know a guy that flies extreme 3D with electric and nitro helicopters up to a TREX 700 on a DX7 for years and no reported problems. So I think the DSM2 works fine. He also flies at a popular field with lots of other systems going simultaneously.
I bought the DX8 and haven't had a single failure. I use Orange RX's in 4 of my planes and have flown them as far as I could see them and never had a problem. I also fly near our local airport and in my experience if you're going to have interference issues, it will be near an airport.
The reason I bought my DX8 is I started out with horizon products and wanted to stick with the BNF capable aircraft. I wanted to be able to fly everything I own on one transmitter. I had also never experienced anything that would deter me from sticking with Spektrum.
The aircraft I fly are in my signature. All have spektrum or orange receivers. I have experienced good things with my Orange receivers but at the same time, for my nitro planes and my helicopters I spend the 60 bucks and get a good spektrum DSMX receiver. Just makes me more confident. Confidence that it will work is worth the extra money.
As far as compatibility, my instruction manual states that all DSMX radios are compatible with DSM2 receivers. They simply revert back to DSM2. My DX8 says right on the screen which mode it's in. All of the Orange RX's and all my ultra micro's are DSM2 and the DX8 works fine.
The only guy I know that had trouble with a DX8 was a guy I met at the field in Fairbanks and he had a $13,000.00 jet with a kerosene turbine. He said he had lost confidence in his DX8 so he got a JR11x. He's still using all the same radio gear in the jet, but he likes the 11x better. It's all DSM2.
Hope this helps.