I use Closure for everything, which is too big for jQuery. Compared to its next best competitor YUI, it's a joy (eg. first really good cross-browser richtext editor).
I have not found many features, not already included in the library.
Code can be easily scaled, and is fast enough. Especially on the production system, where you, thanks to the Closure compiler, can have a compiled version (I also prefer the compiler over YUI's).
Have I told you about the excellent testing framework...
Have I told you about the excellent documentation...
Have I told you about its very readable code...
When it was released, and I had read some of its code, I knew I wanted to use this at my work as soon as possible. But exactly this Blogpost had a super high google rank for the query "Google Closure".
If you, too, run into the problem of your co-workers reading that post, just link to the HN-Comments. Worked for me. Here is the older HN-Link: http://news.ycombinator.com/item?id=937175