Popular I would say it is not, probably because it is java based and just about everyone hates Java. Once you get over the learning curve and negative Java bias I think most will discover it is worth it though.
GWT 1.5 is stable enough in the trunk, I actually recommend using that even though the RC is not out yet (probably late summer at the earliest I've read). There are instructions on how to build it on the GWT site, its very easy. And then you can use Java 5 syntax which helps out greatly when dealing a lot with RPCs.
Also check out gwt-dnd, its a fantastic library which enables drag and drop. Its leaps and bounds ahead any of the other drag drop libraries, definitely recommend it. Fred is the guy in charge of it and he is a great resource: http://code.google.com/p/gwt-dnd/
I've been using GWT Widget Library for animations. Its just a simple wrapper around Scriptaculous libraries. It has some other functionality too which I have not explored. Animations of existing widgets are super easy though, and can make your UI far more dynamic.
Haven't visited GWT-Ext in a while, but last I checked it was coming along pretty well. As far as drag/drop functionality goes though, you will be able to build more custom implementations with gwt-dnd. GWT-Ext just seemed a bit "vanilla" to me since it looks a lot like a lot of other Ext sites out there. We just preferred something that felt more custom, but I guess thats just a matter of preference.
If you code in Eclipse, I'd recommend checking out GWT Designer plugin from Instantiations. Unfortunately its like $30, but if you end up doing lots of coding it will be well worth it. I use it to mock up layouts initially, and its also great at handling RPC code generation. There is a new free GWT plugin for netbeans 6, but its not quite the level of GWT Designer yet.
GWT is great though because it is so modular. As new libraries come out you can just mix and match to get the widgets you want. Try that in native javascript on a large scale and you can quickly get a headache..