The basic flashes, of course, have a very basic user interface. The Pentax AF200FG, for example, has an a power switch, a dial for exposure compensation (or manual mode), and test button. Or the Metz 36 AF-4: there's a power switch and a couple of lights, and the zoom setting is changed by just physically moving the reflector. The Promaster 5750DX doesn't have many controls, but does have a old-school mechanically-operating exposure calculator built in.
When you get to the advanced models, there's a lot more possibilities, so the user interface is much more important. As with the interface of camera bodies, this is highly subjective and one person's favorite feature can be another's pet annoyance.
Pentax addresses the problem directly: lots of buttons, switches, and dials. Most only do one thing, and the buttons with multiple features are next to a slider switch which acts as a function shift. There's not really much to complain about.
The interface on the Metz 48 AF-1 and 58 AF-1 is likely to provoke the most strongly divided reactions. Rather than having dedicated buttons for each feature as the Pentax flashes do, Metz uses a menu system.This means you never have to remember how to get to a certain feature, because they're all accessed the same way. And, this is why Metz is able to cram in so many little nice touches — they don't need to add a new button for each one. On the other hand, this means making several button presses for most operations. My main complaint is that on the 48 AF-1 the parameter menu is activated by pressing two buttons simultaneously, and it can't be done easily with one hand. The 58 AF-1 works a little more elegantly.
The Sigma EF-530DG Super has what I find to be an over-cluttered and non-intuitive interface. I had to keep going back to the manual whenever I wanted to do something I hadn't done before, and even operations like setting an EV compensation value are more arcane than they should be. This isn't helped by the way Sigma uses different terminology than Pentax — for example, the HSS mode is indicated by the letters "FP". I had originally assumed that this was due to simply sharing a user interface with the EF-530DG ST but cramming in more features, but in fact the ST version of the flash has a very bare-bones interface, so the blame has to rest elsewhere. I'm sure that one could get used to the controls after not very long, but if you use certain flash features only occasionally this may be a real concern.
