It depends which platform. The iPad UI is great. The Web UI is ok. Xbox 360 UI used to be ok (I had minor quibbles with label being cut off for tv shows) - but now it's worse (auto-play sucks for tv shows).
Do you have specific complaints?
I personally think the Netflix UI is significantly better than the UIs of Amazon or HBOGo. And on the Roku, it's orders of magnitude better.
For me it's not the UI, but the search and discovery features. Why can't I search for an actor's name?
You can on the website.
Sure. The whole "hover your mouse over this arrow and we'll scroll the poster art at a rate of about 30px per second" is super frustrating. I actually ended up getting so annoyed that I built a Chrome extension to replace that interaction with horizontal scrollbars.
+1 that was a horrible, horrible UI change. So much so that I wrote them an email just to complain about it. I don't know what they were thinking with that one.
Most of the UIs show the same 30 or 40 movies, over and over again. It's especially difficult to discover anything new or specific on the Roku. A (truly) random selection button or category would be nice (one that ignores the suggestion algorithm). It's astonishing to see how many movies are available from Netflix when you visit a site like instantwatcher.com. You'd never know it from Netflix's own website.