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This is one of Canon's premium flash units and it oozes class and functionality.
Construction: The flash unit is very well constructed out of robust materials, with no awkward hinges that look like they'll break through harsh use, and a weatherproofed and dustproofed (to reasonable level of use, I assume - I have used it in sandy and rainy conditions for short periods and it was perfectly fine, but I wouldn't want to push it to see where its breaking point is). The buttons are responsive and setting manual output, for example, is easy - no re-pressing a button because the first time didn't register, a problem I have had with cheaper manual flash units like the Yongnuo 460-II.
Power: Using AA batteries is handy as they are easily available, and I have a set of rechargeable Lithium Ion batteries which are good for a whole day's shoot on this flash (several hundred shots) with a set of spares. The power output goes up to a whopping 60 at ISO 100, which is great for bouncing flash and diffusing flash as the light that reaches the lens is much reduced by the spreading out with distance - so that big topline power gives you flexibility to light a bigger subject from further away or fill a room with bounce flash. The recycling rate is excellent, though at full power the several seconds seems a little long and can leave your subjects waiting - so I'd like to see a little faster at full power.
Head tilt/rotation: Full 360 degree horizontal coverage and 90 degree upward vertical coverage gives me the full range of bounce options. The head turns solidly and always feels well-locked into position. The downward tilt of 7 degrees feels a little measly, as I shoot macro and would like a little more, maybe 20 or so degrees just to give me that little more shooting versatility, but no other flash unit offers this either so it seems harsh to seriously judge the 600EX-Rt for this.
Integration with camera: Great. The flash uses the latest E-TTL II mode and I've found in full auto it rarely over or underexposes, and when it does the cameras I use it with (5d mkII and 7d) have easily enough latitude to fix it in post as the over/under exposure is never by much.
Additional features: Off camera wireless mode, slave mode and modelling flash I have not used, so can't comment. However this flash offers all three and my experience with this flash as a faultless unit inspire confidence in these areas too.
Included accessories: The wide angle panel is a good extra to include though I don't use lenses wider than the 20mm the flash goes to natively (20-200mm). The flash stand is similarly useful to those who use it off-camera, and it's good to see Canon considering these extras as necessary to include and not selling them separately (as they do with lens hood, for example)
Conclusion: This unit can take heavy use. I have used it for whole day events and I have never experienced a fault, which is of course vital for professional use. For more casual users or amateurs I would suggest considering one of Canon's cheaper flash units with less advanced functionality for much less cash, but if you're serious about flash and want to experiment with the full range of functions Canon offers I'd definitely consider this.