I have to preface this with a request to not have this thread turn into a "which is better, the 540 Pentax or the Sigma EF-500" kind of post.
At the end of the day, I chose to get Sigmas because for the price of one Pentax flash, I was able to get two Sigmas, that had many capabilities, and a decent build quality.
Trust me, I've held some pretty shoddy flashes in my day (the cheap Bower flashes come to mind).
I had a shoot on the weekend, so just before the weekend I decided to try out the wireless PTTL capability outlined in the manual. Sadly, my Sigma seemed to be too old, and didn't have the new firmware installed that gave that capability. So, I went out to Henry's and picked up another one, and had the same problem... No wireless. Now, that said, if I had one flash on the camera body, I could control the other one as PTTL, but I couldn't use the built in flash of the K10D to fire them.
I'm lucky enough to be about 25 minutes from Gentec, the place that Sigmas in Canada are repaired at.
I made a quick call, and later that day was at their doorstep with my flashes.
And for no fee, the upgraded both the flashes with the new chips, and after a quick test in the store itself, I found that they worked great!
In fact, I was able to put both flashes in various places in the room and get different shots accomplished.
The recycle time between shots was fantastic, the flexibility of ev Compensation was easy to dial in, and over all I was more than pleased!
It's great to finally have them working as wireless... it really does open up the flexibility alot!
(otherwise, I would have to set up dummy slaves and use the light meter to gauge lighting, but I must admit, I was pleased with the remote flashes with PTTL)
So again, I'm very pleased with the Sigmas, and even more impressed with their absolutely quick, and beyond friendly service. I have to say, dealing with the people at Gentec was an absolute pleasure!