If you strongly restrict mutability (significantly facilitated in Scala with case classes), then OO and FP dovetail quite elegantly. The Kiama language processing library is a fantastic example of getting the best of both (there are many others):

http://code.google.com/p/kiama/wiki/Dataflow

That said, one of Scala most compelling (business) features--great Java interop--is also it's greatest liability. While I appreciate the great interop, Java's lack of state mutation controls in the language and JVM instruction set interfere greatly with the FP/OO impedance matching, generating misconceptions about the viability of hybrid FP/OO approaches.

From 1994 to 1996 I went from being a 80%/20% C++/Python developer to 90%/10% Java/C++ developer. My productivity went through the roof, but the lack of something akin to C++'s `const` references and `const` methods were a glaring mistake, one that remains to this day a shadow mandating contorted defensive programming techniques. Adding `final` almost made it worse, as it's nuanced and overloaded, and ultimately doesn't do what less-experienced developers think it does. Because of that one serious flaw, was I ready to go back to C++? Of course not, because I was able to render my ideas into working software at a faster pace--an extremely important factor--but I had move forward being ever aware of the language weaknesses and how to effectively ameliorate them.

Over the last year I've gone from developing in Java 90% of the time to Scala 75% of the time. In this transition I have seen a similar jump in my productivity (almost, but not quite as big as C++ to Java, and after a longer learning curve). However, have approached it with the same multi-dimensional awareness of one's paradigmatic assumptions and how they play with and against the language facilities.

For me, Scala was my gateway into the world of FP thinking, which radically changed the way I think about software problems. Scala has also had a profound impact in developing a deeper appreciation of the power of a more formal type system. Both of those paradigm-shifting features of the language have allowed me to be more creative, expressive and concise in my software writing, with bountiful rewards on multiple axes.

However, I've also stumbled along the way--becoming enamored of features I didn't fully understand, being too expressive when simplicity would suffice, being FP for FP sake, etc.--but I sure am glad I had those opportunities to stumble, and do so in a "fail-fast" manner. The process has been invaluable, and I'm a much better programmer today for it. I never entered the process assuming the FP/OO/CT academic visionaries or the Smalltalk/C++/Scala/Haskell/ML/OCaml language inventors offered me any "promises". They gave to the world constructs for others to think about and solve software problems, take it or leave it, to live and die in the ecosystem of ideas. I know it is my responsibility as a professional programmer to understand the pros and cons of those constructs and tools, weigh them against my goals, experience and intelligence, and go into a relationship with these tools knowing, I'm ultimately the one responsible for the final product, and need to know what I'm doing.

All this is to say, I don't think sweeping generalizations nor pointed nick-picks help in assisting people select the best language and paradigm for their problem at hand, understanding the strengths and weaknesses, and how to manage those trade-offs. It isn't, and doesn't have to be a "one-size-fits-all world" (as someone else here already referenced the great Stroustrup﻿ quote: "There are only two kinds of languages: the ones people complain about and the ones nobody uses."﻿). And at the end of the day, the ultimate responsibility rests in the hands of the individual professional developer. In my developing awareness of the FP viewpoint I have most appreciated and benefited concretely from those pragmatic viewpoints in the middle. It is from the middle that one can more clearly see both perspectives, and from that develop a third, more holistic and encompassing viewpoint that harnesses the power of both.