A couple of days in I have some further thoughts:
Speaker placement is a total non-issue for me. The camera bump allows for enough clearance that sound doesn't get muffled. And, in fact, it's probably better located than the speakers on my old 920. Any time I played games on that phone my right hand would obstruct the speakers.
Regarding OS stability, overall it's been pretty good and it feels quite responsive. That said, I've had settings and messaging crash on me a couple of times. It almost always happens on opening, popping right back to home. It happens once and then opens fine on the second attempt.
Playing around with the OS the phone gets warm. Cortana, in particular gets the phone quite hot if you're using it a lot. I haven't tried many games, so I don't know what the impact there might be. My 920 never got hot within the OS, but with some games it warmed up quite badly.
On the 950, however, I've noticed a side effect where the display starts acquiring a faint pink glow towards the bottom end of the phone, where it's hottest. It was only evident in a dark room, on a black background and it disappeared once the phone cooled down. I think this sort of thing may have also happened with the 920, but it seems especially pronounced with the 950.
Connectivity with my Band has been the biggest source of frustration. Most of the day I haven't been able to maintain much more than basic functionality. It's like the communication is only going in one direction. The Health app is collecting data, but the phone refuses to accept most commands from the Band. I never had a problem on my 920.
After resetting the Band, reinstalling the app, and deleting the Bluetooth profile a couple of times everything finally seems to be working properly. Hopefully it sticks.