Note: if you came to this page actually looking to get documentation and support, you're not quite in the right place. You may find your answers somewhere on this site, though. If that doesn't help and you're not able to get information by contacting the manufacturer directly, try the unofficial Pentax flash forum at pentaxforums.com, or ask a question on Photo Stack Exchange.
Quality of Documentation
Metz, Pentax, Promaster, and Tumax all make their product manuals readily-available on their websites. Sigma's US web site does not, but the Japanese one does.
Metz deserves a commendation for publishing the most useful timing data, and providing first-rate documentation in general, both clearly written and clearly translated into English. The 36 AF-4 manual suffers from an attempt to document Canon, Nikon, Olympus, Pentax, and Sony variants all at once. Theoretically this model, which Metz touts as having "likeably easy operation" is simple enough that the documentation isn't really necessary. The 54 MZ-4i is much more complex, though, and its manual suffers the same problem, made worse by the fact that you have to also have the SCA 3702 manual on hand for cross-reference, and even worse by being somewhat out of date. The Pentax-dedicated 48 AF-1 and 58 AF-1 models, though, get current and Pentax-specific manuals.
The Pentax manuals are also good and (unlike those of the other brands) feature many illustrations and diagrams. However, some features are inadequately explained, while other pages seem a waste of paper — like the one with 28 pictures of the LCD panel's zoom indicator showing all the possible numbers.
Sigma's manual for the EF-610 DG ST is two pages of fine print in 12 languages, with some diagrams and tables crammed into the sides. There's not much to it, but even with the flash's minimal user interface, it's helpful to learn that MH means full power and ML means ¹⁄₁₆th power, or that the angle indictor blinking 24mm and 105mm slowly means that the wide-angle panel is in use.
The manual for the EF-610 DG Super is more passable, featuring multiple pages, but is still short on detail, and is sometimes comically translated from Japanese. ("The furrow of the subject will therefore be exposed behind the subject, creating a more natural effect.") I also found it the most necessary manual, because the Sigma Super packs a lot of functionality into an arcane user interface and I had to keep referring to it in order to remind myself how to do things.
Promaster produces short English-only manuals which are light light on detail. That's probably okay for the light-on-features models, and even the 7500EDF is straightforward enough that the manual won't be needed terribly often.
Tumax has fairly decent one- and two-page manuals available for their flashes. They're short on technical details but do pack in a lot of information. The English translation is awkward but understandable. Many rebadged units simply reprint this, but Cactus has their own booklet-style manuals as well. (They don't say a lot more than the Tumax versions, but are definitely prettier and feature more natural English.)
E-Mail Support Responsiveness
In the course of research for this article, I contacted all of the flash makers with a variety of questions. Pentax, Metz, Sigma, and Promaster were all reasonably prompt and helpful.
Pentax tech support is reasonably good, but be aware that they will refuse to answer any questions about camera compatibility with third-party flashes — fair enough. Also, several of my requests for more detail have been met with the explanation that Pentax Japan has not provided the wanted information to Pentax USA. I actually contacted Pentax Japan directly too, but that didn't help either.
The Metz technical support engineer was particularly accommodating, even performing additional testing with an oscilloscope when the data I asked for wasn't immediately available.
Sigma response time seems to vary — some questions were answered right away, and others have yet to get a response.
Queries to Promaster's "Flash Expert" via their web site are met with an automatic e-mail promising a response within 72 hours, and indeed, replies usually came back well before that. These were very helpful for basic questions, but Promaster was unable to provide technical information beyond that listed in the manual.
For the Tumax flashes, contacting Tumax directly has been fairly effective. Some messages get no response, but others are answered promptly, and those usually with helpful information. I've had no luck getting any response from Sakar, the holder of the Vivitar brand. On the other hand, the Cactus customer support people at Gadget Infinity were both prompt and able to quickly respond with technical information.