I first became acquainted with Orion here on the dirve, but then I got hired by a software company to do Java development. To my surprise, they were using Orion, too! We've developed a large distributed app with servlets and EJBs using orion as the primary App Server. As we've gotten closer to market, we've started porting our deployment to some bigger names like WebLogic, but mostly we work on Orion.
Tomcat, however, is more of a reference implementation. There are a lot more bugs in it than in Orion. In fact, Oracle 9i Server, which is trying to be a big player, is really just Orion that Oracle is licensing.
So, by using orion here, we're getting experience with a free, standards-compliant system that is becoming a big player in the market.
I'd like to keep Orion.