I use and enjoy the original ASCII, but I don't mind graphic creature tilesets either. I'd never go with a full conversion like Mayday's pack because it messes up the text in menus and dialogues, plus I don't like the look of any of them that I've seen, but once full graphics support is in someday I'll probably start looking for a tileset more in earnest. I use so many custom creatures, however, that it'd be a bit of a hassle. 'Course, I've made a few tilesets of my own, though they didn't look very good so I trashed them afterwards. Sooner or later I'll do some that hopefully are a bit better.
My main issue with DF's presentation is the way the window sizes, which I presume is better in the 40d# versions but I've never used one. Fiddling to make all the resolutions right frustrated me because, perhaps because of a math error on my part, I never managed to find a resolution that actually fit properly on my widescreen monitor. Plus I could never figure out how to make it just show more tiles on the screen, rather than making the tiles bigger, but that's probably my own impatience shining through.
The UI in Fortress mode doesn't bother me. Other than it being awkward to remap if you're using a laptop keyboard, because of the inconsistent scroll-keys (sometimes the arrow keys scroll both columns, sometimes the numpad scrolls the second column...!), I haven't had any particular problem with it.
Adventure mode drove me a little bit nuts because of the lack of any indication of what menu options there even are (I didn't know you could choose what wrestling moves to do until I just happened to read someone mentioning a "spacebar menu" on the forums, totally at random.)