Well, I shoot Canon but I'll chime in here. I've got two 550EXs and a 420EX, so it's a fairly similar setup to the one flosser suggested. I replaced the 420 with the 550 as my main flash when I first started going off-camera (I got a 5 meter ETTLII cord from eBay to keep the 550 on a stand & the weight off my wrist). Getting the power balance between the two different model flashes right proved to be a huge pain, so after the first gig I did with the 550/420 setup I added the other 550.
The 420 is useful in situations where I need a weaker 3rd light for weak backfill or other such roles. Most of the time it stays in the bag as two 550s on umbrellas pretty much does what I want most of the time. I definitely recommend having your primary lightsources be the same model.
An argument for having the sb600 rather than the 3rd sb900 (in addition to cost) is that when you need an on-camera flash, it's smaller, lighter, and easier to use. I'm looking at the back of the sb900 & it reminds me of the hp48g calculators my buddies & I got for calculus class in high school; officially because we "needed" them, but in reality because you could play Doom on them head-to-head over the infrared port 🙂.