Let's take a look:
The SB900 costs $459.95 and the SB600 costs $219.95 at B&H as of this date, for the USA models. The difference is $240.00.
What does one get for that money?
The 900 has a rotary power switch, instead of the push and hold power button on the 600 or 800.
It now tilts down to -11 degrees. My 800 tilts down -8 degrees and I was unable to determine the declination on the 600.
One gets a set of color filters and a diffuser dome.
A longer claimed range of 17 to 200mm, instead of the 600's range of 24 to 85mm.
It weighs more, 14.6 oz versus 10.6 oz.
It is physically larger.
It has lots of menus.
It functions as a wireless commander.
My critique is as follows:
The rotary switch is a positive change. It is usable with gloves and more positive in function.
Increased downward tilt to -11 degrees? If one is shooting close enough, one should consider use of a macro flash, a lightbox, or studio lighting depending on circumstances.
One can buy sheet gel filters for $6.99 for a 20x24" sheet and a diffuser dome for the 600 for $9.99.
Its longer range is questionable, since the light output is about one stop greater on the 900. If you need a longer range for a 300mm or longer lens, you can spend $38.95 for a FX4 flash x-tender for the 600 or 800.
More weight is a burden and a 40% increase adds up when having to lug it and the camera any length of time.
It is larger and if you are carrying it as a PJ or as an enthusiast around at an event, it is likely going to get caught up in things and bumped around or off. Hot shoe damage isn't cheap - I know from practical experience.
It has lots of menus. I prefer simple to complex settings and while they may be customizable, how many customizable settings does one need when shooting anything? Especially if the action is busy or fast.
It functions as a wireless commander. Does anyone routinely use their SU800 or wireless commander? If you do studio work, studio strobes or hot lights would be better. My SB800 has that function and I tried it once. It was a lot of effort for not a lot of light output.
Everyone's needs are different and some people prefer to have the latest and greatest devices out there, but a savings of $240 is substantial and one can spend an additional $45.94 for a big sheet of warming gel [a rubber band will hold it on the flash nicely] and a flash extender for the 300mm or longer lenses, the SB600 can be a serious alternative and still save $194.06. If you want the wireless commander, B&H has a used SB800 [rated at 9] for $349.00 and that is a savings of $110.95 over the SB900.