***Don’t have both a *.xaml User Control and a WinForm that contains the the control inside an ElementHost in it at the same time.***
Here’s what I get: 
Left Image: after restarting VS, you’ll get this message.
Right Image: When viewing your WinForm with the ElementHost that has the *.xaml reference. You can see that it is disabled.
All you have to do is close VS down and fire it back up again.
(Anyone else run into this issue? It’s annoying.)
What are you using their controls for, the web or desktop?
We’ve had a horrible experience with their Web (ajax) controls, I recommend against using them.
Yeah, my project eats up a lot of my resources in design time. I now close everything else before actually working with VisualStudio + ReSharper + NETAdvantageWinForm + NETAdvantageWPF.
It’s not bad after I get over the hogging resources part. It’s a trade off for really bad ass graphics off the shelf instead of creating custom ones from scratch.