Upload & Sell: Off
For the options (bouncing off of a wall or through a diffuser for softer) that it gives over the SB-600, and the MUCH more user-friendly interface, my humble opinion is that it is worth it.
However, you may also wish to consider the SB-700, which replaced the SB-600, or even a used SB-900, which is going for $325 to $375.
2) Big- not just more girth to pack, but also big enough that when I mount it to a PW TT1 on my D700, it makes it harder to hold stable to prevent shake. So I use a PW TT5 w/ an SB-700 instead.
3) Can heat up quicker than most flashes when fired continuously at high power (dark room, small aperture, etc.).
Differences between the 910 and 900:
-Hard Tungsten and Fluorescent gels included.
-910's buttons light up- a big help when shooting in dark wedding reception halls.
-When it overheats, it just slows down instead of cutting off completely.
-Slightly more user-friendly, though the -900 is also very easy to use, nowhere near the user-hostility of the SB-800 and SB-600.