Any downsides to using ASP.Net AJAX and JQuery together

We have used ASP.NET Ajax, jQuery and Telerik components on a large project for quite a while and haven't had any issues

I would definitely recommend using jQuery

jQuery has a noConflict() method as a part of the core, but it then requires that you either use jQuery as your named function or something else of your choosing (instead of the dollar selector). However, I will say that the method is often dependent on the implementation of the "competing" library. I have tried to use it for a Ning social network (which used Dojo), and for Magento (which uses Prototype), and I could not get either to play right with jQuery. This is just my personal experience, and others have been very successful.

http://docs.jquery.com/Core/jQuery.noConflict