I'm an avid user, and my main complaint is, precisely, the UI. Sure, some of the main functions (e.g. traffic report) can be done fairly easily with one hand, but almost everything else is terrible:

- Choosing a navigation destination (slide, pick one from the list, wait not this list - my favorites, search - wait, that doesn't do what I think it does, scroll scroll scroll...)

- Closing those damn popups with the tiny X that is almost impossible to hit (at least on my 3.5" iPhone)

- Pretty much anything involving panning around the map or zooming in/out. It's like the gesture experience is way worse than anything else on iOS, and you definitely feel it.

And some others I can't think of right now. So if one good thing comes from this acquisition [1], I hope it's a massive UI overhaul.

[1]: I'm dreading it, and I fear what will actually happen is the integration of the social network into Google+, and the death of the product itself, somehow subsumed into Google Maps.