My 2 cents: As a potential contributor to the opennlp.ml package with a good background in machine learning, I would say Scala is not a barrier for me (even if I don't use it often right now). Maybe even the opposite as I find coding in Scala more fun than Java. Profiling and perf tuning can be a bit harder though.
The most important drawback I had against Scala in the past was the poor / buggy Eclipse support for Scala which made it painful to work with multi-language (Java + Scala) projects but the situation has very much improved over the past 2 years. -- Olivier
