I've only ever used the default UI. My only complaint is that I can't rearrange the bars... something that, UI addons or not, Blizzard should have changed years ago.
Most UIs are trying way too hard to be flashy and that is never appealing to me. In that sense, ElvUI (by default) is at least okay (from how it looks, I've never tried it). But, I'm comfortable with the default UI and I've yet to feel directly held back by it in any way.