For what it's worth (possibly for folks deciding whether to jump on the iOS15 beta train), here's another datapoint.
I installed PB6 yesterday on my 12PM, from 14.7.1.
A lot of weird behavior during the first 30 minutes, but after a couple reboots and reset network settings, everything was fine. My banking, credit card, and utility bill apps all work fine. Social media apps work fine. CarPlay works great, phone calls work well whether on cellular or over wifi. Wifi and bluetooth are great, with all my devices (headphones, car, apple watch, etc). I haven't tested 5G because in my area, Verizon LTE is as fast or faster than Verizon 5G so I leave cellular data set to LTE all the time. Performance seems as good or better than any version of iOS 14 I've used.
In short, if you're on the fence about installing iOS 15, I recommend it. Just make a full backup to a computer using a cable so that you can go back if you have a bad experience on iOS 15. But I think that's pretty unlikely at this point.
You can always wait a week for the next beta, which will be a bit more refined and even less likely to have show-stopper bugs.