We have started to achieve some really good results by integrating jQuery and ASP.NET. I wrote a post back in August 2010 that described my confusion about how best to make the two work together. After some more work we have found that the two technologies complement each other very well - it's possible to create a slick UI and also rely on the data management capabilities of the .NET framework.
It has also begun to alter the way that we work as a team. There is a definite seperation of duties between the UI design and the data transaction design. So now we can have one person working on the look and feel, and when they're happy someone else can follow on behind them and fill in the data 'blanks' - such as making sure that the login process works OK, or making sure that grid views of data are displaying properly. That sort of thing. The division of labour also leads to increased productivity - Adam Smith was right.
The switch to jQuery also means that we can do away with the .NET AJAX control toolkit. I'm really glad to see the back of it. It was inflexible and rigid - the total opposite of the jQuery library.