I've purchased the first one about 3 years ago and, once I learned the menus, particularly, the "wireless" modes, I was able to get great results under difficult lighting conditions shooting on location at a small manufacturing facility. Using the built-in flash on a K-10 with the '540' set up as a 'slave' unit bounced into an umbrella, I was able to get all the shots needed to show this company's capabilities--one of the machines was over 16' long and I was able to light it end-to-end using this setup.
I just recently found a used one on eBay and was the high bidder until the last second: another bidder punched in a higher bid and took this unit for $329.50. A new one at B & H Photo sells for $389. I bought it.
The door issue some have complained about
: I don't see the problem. I treat all camera equipment carefully so to me, this flip-open door is not really an issue. I just don't get rough with it.
I'm looking forward to my next assignment: shooting promo photos for a local professional harpist. The two '540s' will be used in combination as slaves with the on-camera flash being used as "control" to provide soft side lighting from one direction and rim lighting from the opposite side. I'll use either one or two umbrellas to soften the affect--or perhaps the secondary slave will be bounced into a folded white foamcore panel. The mode for both will be PTTL.
I shopped for the Metz and I've heard good reports on them. I just felt a flash that's dedicated to the camera in my hand (K5) would be a better choice.
Now the choice is yours.