I've had a Metz58 flash for a year and a half and received a Pentax 540FGZ a few months ago.
I wanted to test various combinations of these flashes, see the effect of contrast control, etc.
First some remarks not related to the flash behavior:
I like both units, the bodies of both feel sturdy. I would perhaps rate the feel of the 540 flash head higher than the Metz. On the other hand the 540 has a battery cover which feels fragile, I operate it with care. Also the control sliders of the 540 feel less solid, but not in such a way that I feel that they are likely to malfunction.
Completely subjective: the 540 has slightly better looks.
You can find the complete 21 page test report in PDF form here (Updated June 2009).
Contrast mode works, but not in the way I would expect, c.q. wish it to work. The flash with contrast-on emits less light, but contrast-on does not work for the on camera master. So one cannot have the on camera master act as fill.
Contrast mode implementation on the Metz and 540 are not completely similar. The 540 produces slightly different results (lower ratio for the off camera slave?) than the Metz as off camera slave.
When using 2 slaves the contrast-on option gives inconsistent results. If the Metz has contrast-on then the option has no effect. If the 540 has the option on then the Metz does not fire.
In a controlled environment one should use manual for really predictable results.
In a dynamic environment with a master flash on the camera one should move the slave flash away/towards the subject to change the master/slave ratio or play with exposure compensation on both flashes.
Both the Metz-58 as the Pentax 540 are really nice flashes. I think I prefer the Metz as on camera bounce flash with the option to use the second reflector as fill. The 540 is really good as off camera flash and even has an optical slave mode.
I also considered the Metz-48 and Pentax 360 flashes. They are good options as long as one is not interested in using multiple flashes. The drawback of the Metz-48 is that it cannot act as a master/controller on camera, otherwise it would be a great second flash for me. The main drawback for me of the Pentax 360 is not that it doesn't have the swivel head (I assume I could live with that), but that off camera the automatic power off option cannot be disabled. I use off camera flashes a lot with optical or radio triggers and having to turn the 360 manually on after a few minutes (and restoring all its settings) is no option for me.
See Mattdm's site: http://pttl.mattdm.org/
for more flash options and all kinds of details about the flashes.