SB600 does not have optical slave but u can trigger with ur pop-up flash from models such as d90.

addin to the list:
6) SB600 has FP mode which allow you to shoot at speed higher than x-sync

I have used both and I wont buy another di622 is because
1) it does not have FP mode which means that I can only use it at max x-sync speed. (a big turn off when using it as fill-in flash in outdoors)

2) the TTL isnt that accurate and most of the time i have to use manual power instead.....

3) the annoying beeping noise emit from nissin when you switched it on.