The N91 is a great music phone if you are a power user or geek. Why? Well because it doesn't "just work" with iTunes which is what most people are using. With the Nokia Music Manager on my Mac, I can sync playlists (but not smart playlists) manually from iTunes. I, as the user, have to manage what will fit on the N91 rather than having iTunes do it like it can do with smart playlists. So all in all not for normal people who use iTunes. Need seamless iTunes integration for normal people!
If you don't use iTunes and you don't mind drag and drop (which is less than 10% of the world I guesstimate!) then the N91 works fine. I dragged 2.5 gig of random music onto my N91 last week and it was fun listening to it.
Having the phone and your music is very compelling. After my week of having my music on my phone, an iPod integrated with a phone done right is inevitably in Apple's future methinks; Apple is of course biding their time and waiting for an opportune moment.
It's a no brainer that the phone and music player will converge especially if the phone has a standard headphone jack and (for those with money) supports stereo Bluetooth headsets.
My prediction: the Pop port will go the way of the Dodo and by 2010 all phones will have normal headphone jacks and all handsfree talking will be done either via speakerphone or a wireless headset.