I have been using N 6680 for the past three months.

Before I upgraded the firmware:
Positive sides:
1. Excellent speakerphone feature. The speaker's voice is clearly heard
by the receiver.
2. Excellent organizer software which can be synchronized with your PC.
3. You can install additional S60 application to fulfill your personal
requirement.
4. Features rich.

Negative sides:
1. Basic alarm.
2. It plays MP3 but the sound quality is poor even if I use its earphone.
3. Sluggish (typical of a symbian phone) - not responsive
4. Prone to crash.
5. Button 2 is too closed to the Navigation button.
6. Camera won't work as expected. It keeps on complaining of low memory.
7. No infrared.
8. No FM radio!

I downloaded Nokia Updater software from Nokia UK website and updated my
phone's firmware myself through the Internet. After I upgraded its
firmware from v4.04.07 to v5.04.07 15-02-06 RM-36:
1. The phone is slightly more responsive (I think).
2. No more 'low memory' error.
3. It will just restarted itself when it crashed. Rebooting time is less
than a minute. Fine for me.
4. It works perfectly as expected until today.