Ask HN: Single-page Apps... What's your experience with GWT?

I am recently beginning my search for a good library to make a single-page application. There were several options including: sproutcore, cappucino, extjs, and GWT. Of all the ones I've reviewed, GWT seems to have stood out. Cappucino is great, except the one component that I'm looking for (a rock solid TableView) is not available.

Sorry for the long wind up, but my questions are: - What is your experience with GWT? - Have you run into any "leaky abstractions" while using it? - How easy is it to customize/skin the UI? - Any ideas how this project was able to skin the UI: http://vaadin.com/

Thanks for your input in advance.