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 higher-end Metz flashes 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. Unlike the Sigma user interface, the operation of each menu function is consistent, so once you learn what each option does, you don't need to learn how to set it. On the other hand, this means making several button presses for most operations. My main complaint is that on the 48 AF-1 (and 50 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/AF-1 works a little more elegantly.
The Metz 44 AF-1, on the other hand, has a very straightforward interface with a small number of elegantly laid-out buttons and lights. None of the controls do anything tricky — the most complicated is that the exposure LED blinks to show that a lens is wider than the zoom reflector can support. There's nothing else non-obvious. This is unique in a flash with advanced features like wireless P-TTL, but the design decision means that some features which the flash nominally supports (for example, rear-curtain sync and modeling light) can't function, and that the wireless mode fires on all channels (since there's no way to input a number).
The Sigma EF-530DG Super and EF-610DG Super have 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. (The ST has its own confusing touches, with full power indicated by MH and ¹⁄₁₆th power by ML — "manual high" and "manual low", which is memorable enough once you know but not outstandingly obvious.) Anyway, 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, and I found it frustrating.