You don't say if you've got any background with Java. If you don't, then, well, I can't speak to what your learning curve may be.
However...as someone who's been working with Java for ~9 years, and much of that time spent with Swing, I've found GWT very easy to learn.
The widgets are different from Swing, so that takes some getting used to. But, from my perspective, it's really no different than learning any other library.
Personally, I love that I can use IntelliJ (my editor of choice) and take advantage of all the fantastic Java tools that help me write better code. (BTW...that's part of the GWT mission.)
I love the fact that this is a vibrant, and highly active toolkit, with lots of people really interested in making it better. (Again, take a look at the discussion group, or even browse through the Contributor's discussion group.)
If you want access to more or different widgets, there are lots of projects looking to fill the gaps:
(NOTE: I am NOT endorsing any of these project or commenting on their relative merits, just trying to provide some references...)
I think if you dig around on the web, especially taking a look at the GWT Discussion Group you can get a good feeling for what others are doing with GWT.
Having said all that, beware...you will still find some issues with how things render on IE6 vs IE7 vs FireFox 3, vs... I've not written a webapp in straight JavaScript or with any other toolkits, so I can't say how much better GWT is in this respect. All I know is that in 10+ months of working on the application I work on, we've encountered only a few cases where we had to specialize CSS for one browser or another.