The downside to Drools is that the framework is written in Java. Sure, Mark Proctor and gang have put a lot effort in making the platform accessible to other languages through web services, but this is far from the level of integration I would enjoy (like a C# version). I think I should take the time an caveat the fact that I consider myself to be a Java programmer (first and foremost) and then a C# developer, but after a couple of solid weeks in .NET, I find it difficult to go back. The one thing I do love about Java (more than anything else) is the language's amazing Open Source community. So instead of embracing the "Dark Side" and becoming a Microsoft evangelist, I have decided to look into the newer languages written for the JVM.
The two JVM languages I find the most intriguing are Clojure and Scala.