"Hi, just want to know if you think an owner of an N91 would feel that this was an improvement over the N91, namely, does it have gapless playback (no blank space between continuous tracks, like classical music or dance music mixes) Of course it seems the advantages are that it is the new FP1 and has a much better screen. Is it an upgrade, an overgrade, or a downgrade?"
Whether or not you want to move to the 5700 from the N91 depends on what features you find most important.
The 5700 doesn't have gapless playback, there's a one second silent gap between two continuous tracks. I ought to add that to the review actually, thanks for reminding me.
However it is a completely silent gap which is an improvement on the old S60 music players you found on things like the E61. 😉
Musicwise I think the most significant upgrade is the 5700's support for A2DP bluetooth stereo headphones, they make it very convenient indeed to listen to music if you have your phone stowed away somewhere, and some bluetooth headphones include built-in music and call controls.
The screen is particularly excellent, the 240x320 resolution makes things a lot clearer on the screen, and many applications and games now demand this resolution in order to function. Video playback on the 5700 is the best I've seen so far on a smartphone. It's also good because the twisting mechanism lets you stand the phone horizontally on a table so you can watch the video like a mini TV.
You can see a video of the video in action on the Gadget Picture Show:
http://www.neoncherry.com/gadgetpictureshow/readarticle.php?article_id=22
On the other hand the N91 does have a 4gig or 8gig HDD, whereas the 5700 can only store 2 gigabytes per microSD memory card. If you use more than 2 gigs to store music, then you might have trouble switching to the 5700. You could of course use several microSD cards with the 5700, as the phone's card slot supports swapping them without turning off the phone, but this is less convenient than just having everything stored on one HDD.
"Or should we wait for the next "music" N series phone?"
Well... the Nseries tag is mainly a marketing thing, and more to do with Nokia's corporate structure than huge differences in Nokia's S60 phones. Nseries phones are made and marketed by Nokia Multimedia, whereas the numbered phones are made and marketed by Nokia Mobile Phones. They're both very keen on making music phones, so you'll see some intra-company rivalry between them over the next few years, especially now that NMP have launched the XpressMusic range (which the 5700 is a part of).
The very latest phones announced by the Mobile Phones division have got compatibility with 4 gigabyte SDHC microSD cards, for example the new Nokia 6500 Slide, so the N91's HDD is becoming less and less of an advantage as time goes by.