Sean Lin has written about how Ajax.NET Professional (AjaxPro) can be used with ASP.NET MVC and what you have to change to get it working.
Anyway, I have been using Ajax.NET since .NET 2.0, as well as since .NET 3.5. I haven't found a good reason to switch and it even runs under ASP .NET MVC and I reckon it is still better than using Ajax.ActionLink(). Unless of course, there's some easier method that I do not know of then please feel free to enlighten me anytime. Most of my Ajax use centers around requesting data from the server asynchronously and then using javascript to update the DOM.
As there are still a lot of developers starting with AjaxPro right now I will publish a new release during the next weeks that will change AjaxPro to use jQuery instead my own JavaScript code.