<p>I was checking out the <a href="http://www.tibco.com/devnet/gi/" rel="nofollow noreferrer">TIBCO GI</a> the other day and I was impressed.</p>

<ul>
<li>Has anyone used it extensively?</li>
<li>What are your thoughts?</li>
<li>What drawbacks did you encounter?</li>
<li>Is it suitable for Internet or only Intranet apps?</li>
</ul>

<p>I appreciate any input you can shed on this.</p>

Yes, I have used it pretty extensively, here are some of my thoughts. Will add to them as the occur.
I don't rate the approach of implementing the IDE in the framework itself (it runs in a browser window). There are many, many quirks and it can lead to a very frustrating experience. I normally develop all of the JS, XML etc in Intellij Idea using the Javascript debug window to run the IDE itself. After working on a file, it has to be reloaded in the IDE, but its still much more efficient.
Using the IDE, an entire application can be layed out relatively quickly.
There is a pretty steep learning curve. The API is extensive and the community forums aren't great when you do run into problems.
Much of the GUI painting is done using inline style attributes, making them difficult to skin and get the overall application looking exactly how you might want. If is for this reason that I think GI is more suited to intranet apps than publicly available ones.
GI uses a 'dual DOM' approach, holding it's own internal representation of each on-screen element. This can make it difficult to fully integrate other frameworks such as JQuery etc.
As RIAD environments go, I think it is awesome. I find that doing things the GI way is an order of two faster than going down the request/response route.
I really like the enforced separation of presentation and business logic.
The JS extensions they put in, like introspective package, interface and class definitions are quite clever and very useful.