What the author calls old habits, I call productivity. A lot of us started off coding imperatively and slowly started adapting functional along the way. It allowed us to get the best of both worlds. Over the past year we've definitely stumbled a few times (mutable objects for akka messages are really really bad), but overall we've been very happy with the stack.
One of the only real ongoing problems we are continuing to struggle with is limiting the impact of big GCs in production. Scala tends to generate more garbage then pure Java, so the GC has to work harder. Most of these objects are in the eden space, but we can run into memory fragmentation issues, which leads to eventual long compaction cycles.
Haskell has relatively simple syntax and coherent semantics (being unencumbered by a 'foreign' VM eco-system, such as the JVM). As such, it is somewhat easier to get into. But to quote Gerry Sussman (co-author of SICP and Scheme), 'Haskell is the most advanced of the obsolete languages'. It gets deep pretty fast. You don't learn Haskell, so much as get initiated into it. It's an ongoing process. This can be said of Scala as well. Neither are particularly small languages when you consider their entire respective eco-systems.
In general, if you are already competent in a mainstream, dynamic language, and want to get your feet wet in functional language concepts, diving into Haskell is not a bad idea; but you are not likely to stick with it. I would recommend Clojure for such people. However, if you are already a Java programmer (and don't completely hate it), then the transition to Scala will be tough, but somewhat gradual. It is designed to be. But I would also say that the Scala world (not so much the language itself) is easier to grok if you know Haskell to at least an intermediate level. I think it is worth the time to learn Haskell and build something useful with it. It does not need to precede learning Scala, however.
Hedge funds? Or something else?
Theres at least 2 not finance enterprise focused haskell shops in NYC that are currently profitable and growing, theres at least 1-2 other places that have a smidge of haskell here and there.
Oh, and theres my place, Wellposed, which is pre launch but for the first product, only Haskell is used, and if it works out, the # of (partially) haskell shops will increase quite nicely :)
I'm not certain if this is still accurate; my information is pretty out of date.
So, we re-wrote it in scala using lift. We were not pleased with the results. So we re-wrote it in scala using play. We were even less pleased. So we re-wrote it in haskell using snap. We were finally happy. That version is now in production, and the type safety of haskell makes doing changes far safer. The maintenance burden is much smaller now.