I have seen demos of ComponentArt's and Telerik's toolsets. I am interested mainly on finding a grid control, a tabstrip control, a menu control and a treeview control
In answer to part 2 of your question, I've used Telerik's MVC controls on a couple of projects now. I've found them to be very reliable and in general easy to use. Two complaints would be 1) not being able to EASILY apply separate themes to multiple controls used in the same project, and 2) the lack of sample code available and sparseness of the documentation.
I recently looked at a few control suites for MVC from the following companies ...
I eventually settled on Telerik as we already have a license for that and they were the nicest looking, easiest to use controls. Also, they rendered in Firefox, Safari, Chrome, IE and Opera Mobile identically.
I had a really good support experience with SyncFusion and DevExpress, both companies were very quick to respond to my queries and one of them (I can't remember which now) actually got a bug I reported fixed in just over a week.
I discounted ComponentArt as the controls didn't render in Opera Mobile at all and also I couldn't get one of the view properties out of the controller code. Maybe I was missing a trick, but I managed it quickly enough with the other 3!
Another deciding factor was that telerik has an MVC date picker and none of the others did.