I'd have to echo most of the above comments. We're in the process of implementing a Sinatra/HAML stack for browsing and basic reporting on data.
I really like the combination of simplicity and direct connection to Ruby. If something isn't working in the browser, it's pretty easy to port it out to a straight Ruby program for debugging.
There's definitely a feeling of building a dog house with Sinatra versus the Taj Mahal with Rails. It's easy to comprehend (it's also helping me understand the need for MVC and Rails).
Altogether very cool stuff and very fast. I haven't stressed it yet, so I can't say how well it'll behave in the real world.
