GWT is great, but I hate Java, so I spent a while messing with Pyjamas today. It is not ready for primetime. I'll give an example. Like GWT, the documentation is in a book reader ("bookreader" means it is an AJAX reimplementation of static HTML -- so you get static pages, but search engines can't index it, you can't easily download it for off-line viewing, and the interface to the web browser is off in little ways). I tried viewing the docs at work. It wasn't compatible with the old Firefox we had there. I tried viewing them in the latest Opera at home. No go on the compatibility front. Finally, I used a recent Firefox, and it worked.
It brags that it cut GWT's 80,000 lines to 8,000 lines, but it appears to have lost a lot of the functionality of GWT in the process. Part of this is that it is a thinner layer than GWT, but a big part of it is that it cut important things.