jQuery UI vs. Zepto

+1 to pretty

Broad Use

Mostly used to provide a basis for needed plugins.

To have fancy effects.

4 years of experience using jQuery UI for front-end coolness.

Front end form capabilities and animations, drag and drop, etc.

The JQuery libraries are embedded in my home page that allow my site to be viewed the way I want them to.

We fully integrate it into the framework and also supply a nice theme.

menu tweaks and animation

Calendar component

In the front-end Angular apps.

basic

Used to easily build responsive modals.

Web UI.

I only use the modal dialog thingy in jQuery UI, which displays when the player is not currently playing. I'm not big on the DOM and so it would have taken me a lifetime to figure out how to do this on my own, as compared to the 5 minutes to install jQuery UI and invoke it.

I use the animations in jQuery UI.

Necessity.

I'll consider Zepto if jQuery is overkill for the amount of JavaScript work I'll be needing for the specific project.

For the occasional lightweight JQuery alternative.