The Canon radio gear is super reliable. We tested the water initially with just two 600s, and we were so happy with it, that we are at 4 600s and the ST-E3.
At this point we have more than 3 months of extensive experience with the system and the verdict is clear: Absolutely recommended.
Pros/Cons of the Canon Radio Setup:
1) Super Reliable. Compared with previous radio option: Pocketwizards (MiniTT1/FlexTT5) controlling 580EXIIs. The interference problem on the 580 seems to be gone on the 600.
2) Controlled directly from the camera. Canon doesn't advertise this as much as it should, since this is a killer feature (External Speedlite Control Menu).
3) Allows for 5 different groups, with up to 16 flashes overall. This allows for complex lighting setups. I believe Nikon only goes up to four groups (if you use the master as the first group, from the mcnally sketching the light book)
4) No line of sight requirement: As with every radio solution, of course.
5) Compatible with previous Canon flashes, using optical transmission. Optical works OK indoors, but is very tricky outdoors, specially if the sun is hitting hard.
6) Good Range: Canon's range estimate is very conservative. You can get more range.
7) You end up carrying less gear and weight around. For events, this is critical.
1) Expensive?. Typically, this is the first complain (it was mine complain at first), but compared with the previous option, a 580 plus a pocketwizard, is about the same. I guess, the Canon marketing guys did their math right.
2) Still, you end up spending thousands of $, when you purchase multiple flashes (For events, is indispensable to have at least 3 600s to rotate them as they overheat)
3) Not compatible with other lighting equipment. I don't know of a way to control and fire other lighting gear (strobes) with the canon system. (If anyone knows of a creative way, please let everybody know)
4) Overheating: When pushed hard, I see very similar behavior as the 580s. Not sure if the way the 600s handle heat is a major improvement over the 580s
5) Can't think of other cons.
Please feel free to add pros and cons to this list.
I would love to read opinions from neuro, spokane and all of the excellent participants of this forum, the real value of this forum.