I have no doubt about a tenfold variance across different people, but I'm highly skeptical of any claims that a particular language will make you 10 times more productive over the long haul.
People always doubt this, and then new languages always come along that improve developer efficiency. It was a lot easier to write a program in C than assembler, Perl than C, and Python than Perl, at least for me. I've seen some amazingly sweet code in some of the functional languages like Clojure and Haskell. My current main language is Ruby, and while it is not perfect, I would never trade it as a general-purpose language for assembly. I'm sure I'm well over 10x more effective than I would be if it was all assembly, all the time -- assuming I wasn't allowed to invent a new language as part of the solution.
So why doubt this same thing can happen again? It's like doubting that MD5 will ever be broken, or questioning whether something like Roman numerals can be improved upon. Of course it can, and will.
Clojure is not a 10x improve to over java, full stop. It's not even close, and I've done a project at my day job in clojure, it's my favorite language.
