I got the chance to play with the Nokia N91 aka the Music One for a few weeks and I said I'd get back to you all with my impressions. I am happy to say that 4GB on a phone is quite a useful bit of hard disk space but that with the added bulk and complexity comes a grave price.
I always reboot my phones pretty regularly. I install something stupid and it kills the OS and I chalk it up to my own ignorance. Unfortunately, the N91 has rebooted a few too many times for comfort. Granted, this is a pre-release unit, and I'm sure Nokia will debug this thing to a fault when it really hits the open market, but it's an interesting thing to know that our phones are now smarter, and BSOD more, than a Windows Me machine infected with porn spyware.
Again, I can't judge this phone too harshly. It does everything right: physical keypad lock switch, lots of disk space, and excellent sound quality. The reception has been good thus far, although the microphone is apparently a little muffled, causing people to call me to ask if I'm in a tunnel. Another annoyance, to be sure, but not a dealbreaker.
Here's the best part: this thing is a real MP3 player. Sure, it doesn't support much DRM, but DRM can sit and spin anyway. As we all know from Jurassic Park, life finds a way, and if I really want to listen to something on the N91, I'll figure out how to get it on there. The dedicated music keys are excellent and intuitive and the music stops and starts when a call comes in.
The N91 shows up as a USB drive in OS X and includes a Windows-based media desktop system. It works well with iSync and has thus far been quite a valued companion on the train.
Overall, I'm giving this pre-release model a 7 out of 10 and would recommend it to anyone looking for a smartphone/MP3 player combo that doesn't dumb things down.