the problems with N91 are:
1) Dust might seep through the joystick hole onto your screen
underneath 
2) You need a pouch to prevent scratches (and dust)
3) It weighs 160g. (N80 is 132g, head-heavy phone btw.)
4) You have to get used to the small keypad buttons (N80 has keypad
ergonomics issues also. N71 wins on this. 
)
5) Battery life is poor if you play video (but not as bad as N80
which can konk out with intensive usage like call+SMS+music in as
short as 3 hours, so u can consider 2 or 3 batteries. N91 can tahan
music player for much longer and the battery bar won't drop.)
6) Back of phone gets hot (due to HDD and WLAN adaptor) if you are
copying a lot of files, playing videos or surfing WLAN for
extensive periods
7) The screen is not as good as N80, but the same as the older
Series 60 phones like N70
8 ) No protection cover for camera lens (same for N80 also)
9) Camera shutter sound cannot be disabled (same for N80
also)
10) Lack of third-party applications, which will take a few months
(same for N80 also)
But N80 and N91 are fast enough to play clear videos without lag.
Very shiok. Now I just need content like Guess show or TV serials
that kind to dump in.
Anyway, about N80, it's like 7650, be careful don't drop the phone
when u press the last row buttons (*, 0, #) on the slider cos the
phone is head-heavy. 
