(Hit the jump)
The YN-560 and other manual flashes excel as a second or third flash if used for setup shots. It's hard to beat the value that the YN-560 offers, especially now that it's been updated (with a metal foot and apparently much greater reliability).
However, I take mostly candid, non-setup photos and I rely heavily on TTL when using flash. For my kind of photography, I've found that a flash that can operate wirelessly on TTL is a tremendously useful tool. While I used the YN-560 primarily for setup shots as a rim light or other accent light, the availability of wireless TTL on the SB-600 meant that I could use it not just for special effects but also to work with my SB-800 as key or fill light. So I've been using the SB-600 far more often than I did the YN-560.
The shot above shows what I mean. In terms of lighting, I used the popup flash (covered with SG-3IR) as commander, then used the SB-800 and SB-600 as slaves in separate TTL groups. I've found that with 2 TTL groups, it's possible to improve upon single-TTL flash image. (That's another topic I will cover in the future). For now, just to show that not all of the result was due to postprocessing, here is the unedited shot:
|original unedited shot|
1. The CLS sensor on the SB-600 is acceptable but is not as sensitive as that of the SB-800 or even the YN-560.
2. Cumbersome controls. It takes more than a few button pushes to operate the wireless menu.
3. No commander capability. Right now I'm forced to use the SB-800 on-camera as commander while using only the SB-600 as a slave. If the SB-600 could have been used as a commander flash, I would have been able to use the more powerful SB-800 as a slave instead.
Well, #2 and #3 in that short list have been addressed in the SB-700. As for #1, I don't know about the SB-700's actual sensitivity, but I'm guessing the CLS sensor sensitivity on the SB-700 is similar to that of the SB-800. So if I had to do it over again, far from regretting the extra I paid to get the SB-600 over the YN-560, I would actually choose to buy an SB-700 instead of an SB-600.