What benefits can Vaadin offer my GWT Appengine app?

Question

The GWT page on the Vaadin website is a bit sparse: https://vaadin.com/gwt

"Only benefits, no gotchas You will enjoy everything you love in GWT and get the additional benefit of a novel server-side programming model, components, tools, themes and other extras. If you do not like what you are seeing, it is easy to return as long as you stick to using com.google.* packages. You will probably be tempted to start using features in com.vaadin.* packages eventually. But there is no need to worry—Vaadin Framework is also distributed under the same liberal Apache 2.0 license"

I like some of the look and feels on the Vaadin website, but I'm nervous about "server side models"... I would rather have most of the code running on the clientside if at all possible, and I'm checking GWT RPC calls again (often with the same java code) for security. I don't like the idea of lots of back and forth.

Given my concerns, is it worth digging deeper into Vaadin? Or do I stop here? Can I leverage the various look&feels without stomaching everything? Any other non-intuitive answers about leveraging would be appreciated.

UPDATE: Please don't answer the question comparing Vaadin vs GWT, offering alternative UI frameworks.

I've also used SmartGWT, GXT, and bundled GWT widgets etc. Also familiar with some really complete widget sets like DevExpress for .NET . The reason I asked the question is because Vaadin LOOKS really cool... I'm looking for answers like : No, it's not possible to extract the L&F from Vaadin without compromising the clientside OR besides the L&F stuff, there is cool validation stuff etc etc etc which you can use, and then perhaps some useful evidence to back up that position (tried and failed).

Answer 1

I am answering only a part of the question about the L&F i.e GUI and Security.

1) L&F Vaadin upto 6.x release does not support independent use of widgets. You need to get into the whole server side models.

However, from version 7 onwards Vaadin is leaning on spinning of Widget side to allow widget use without being forced to use the back/forth jsf like server-side state maintenance. Reference - https://groups.google.com/forum/?fromgroups=#!topic/google-web-toolkit/3U1h0W_iHcM

2) Security GWT side has pretty nice support with XSRF feature for RPC which would allow rpctoken generation per RPC call or selectively with your choice of granularity. This be might be a performance overhead for Vaadin roundtrip per sever-side-state call.

3) GAE is a factor depending on how heavy your Vaadin roundtrips really are.

4) Future

Vaadin is a member of GWT steering committee and along with Jboss Errai (asynchronous bean management in Errai roadmap) lean heavily on server side models.