Let me try to address your comments:
1. What additional functionality would you want to see in a button for ASP.NET AJAX? Generally we find that the standard buttons in ASP.NET AJAX provide all the functionality people need. If you're interested in styling the buttons, you might want to check out RadFormDecorator
2. Similar to the buttons question, what would you like to see from Telerik in the login controls that is not already available from the built-in versions? You can use many of the existing RadControls to customize the ASP.NET membership controls (via their template support), so maybe that's a good option to consider.
3. For MasterPages, you should have no trouble using the RadControls for ASP.NET AJAX. All you have to do is drag and drop our control on to your master page or content pages and everything should work. If you have specific trouble, let me know and I'll be happy to help.
Hopefully this helps a bit. I'm more than happy to elaborate if I've missed the point to some of your questions.