Well, I honestly don't know much about GWT, and have not used it myself. My advice, with any technology, is to try it yourself and draw your own conclusions. Too many times, I've seen comparisons of one technology to the other that were biased, based on one person's (or group's) preferences, that I've always decided to try things myself. For instance, the C++ vs.
Java argument that went on for years, Eclipse vs. NetBeans, Windows vs. Mac OS X, and now JavaFX vs. Flex, or SilverLight, or GWT, and so on. They all seem to have strengths and weeknesses and will appeal to different people differently. I like JavaFX because it's almost like an extension to Java, with a powerful language that makes it quick and easy to build GUIs, and runs on desktops and mobile devices alike. Let me know what you think after you've played with it. Try it at www.javafx.com
-Eric