GWT's downside is that you're now making a rich client, and, IMHO, rich clients are a PITA compared to web 1.0/web 2.0 apps that just generate HTML on the server-side.
A rich client generally means you aren't sending HTML across the wire anymore, you're sending objects (json/xml/whatever), which adds a whole new layer of serialization and deserialization to your app. Even if GWT "solves" the technical side of serialization for you, you're (typically) still getting/setting data in/out of a lot of DTOs.
A rich client also generally means you're doing fancy validation/event driven coolness that, for me anyway, takes a lot more time to get right than just "here's a text box, have fun".
So, yeah, you'll get really slick apps with it, but I would be hesitant to use it if you're not certain you need a gmail-style one-page app.