Alex Peake writes in a comment...
I would love to use Franz Allegro or Cincom Smalltalk in all their maturity. On the other hand, if I have to write my own infrastructure and GUI components, I will never finish the "real" project.So I'll open this up for others to respond. Generally, these vendors (e.g. Franz and Cincom) have to provide connections to Java and dotnet at least as easily as they did for C. Given these systems are run-time typed, reflective, and garbage collected, the implementations should be easier.
What do you do in Franz and Cincom for the Infragistics GUI components, ActiveData reporting, GoDiagram...? What do you do for .NET Remoting, Declarative Transactions, MessageQueue, Declarative Code Access and User Security (based on Windows Login)...?
Franz has a set of tools for Java, and so I would expect something similar for dotnet. Cincom has an preliminary interface for dotnet, with a more complete (events, etc.) interface in a few weeks. I guess for Java, Cincom Smalltalk would use the JNI.
Specific capabilities are probably hit-or-miss. Cincom supports an MQ-Series interface apparently. I guess the question for someone in a situation like this would be does the use of some specific product outweigh the benefit of building the majority of a product in a more productive environment? And what the cost is for creating a specific interface to that product from the ground up.