If your application already uses MS AJAX, I would suggest you begin the process of adopting "Prometheus" RadAjax. Your pages will already be downloading the JS files for ASPNET AJAX, so you might as well take advantage of the resource and use "Prometheus" (instead of also making clients download Telerik's RadAjax client framework).
If you're not using MS AJAX yet, then I'd suggest you start making plans to incorporate it in the next 12 months. "Prometheus" Ajax is the future of RadAjax, so it is important to (at the very least) start planning for its incorporation into your projects.
For complete answers to most of your "Prometheus" questions, do check out the FAQ Leon pointed and see my latest blog post
on migrating to "Prometheus" RadAjax.