Started Using Knockout.js
Back when I was using Adobe Flex to build It's My Party I wrote a framework that somewhat reminds me of Knockout that used data binding and a data access layer to make it easy to capture good input and save it to the database. Knockout reminds me of the data binding portion of that framework and I can tell it's saving me time from that alone.
First impression: it's been handy. So far it's helped provide ways to easilty and intelligently bind sections of the page to various data objects.
Looking forward to doing more reflection and analysis after I finish the project I'm working on to decide where Knockout has been most useful versus where the tradeoffs.