I have used it. I like it quite a bit. It's very simple and integrates nicely with Scalate, a Scala templating library.

Scalatra + Scalate + development using simple-build-tool (using ~ prepare-webapp) is a pretty killer combination, in my opinion. The feedback cycle is every bit as fast as rails.

The linked in engineering team just deployed scalatra as part of their big new faceting search engine.

http://sna-projects.com/blog/2010/10/linkedin-signal-a-look-...

i am somewht responsible for the original idea, though others have done, well, all the work :)

Scalatra has a few rough edges but since it takes about 2 minutes to have somehing running it is worth trying it out yourself :)