> You really don't sacrifice anything by using it

What's "it" - Haskell, or referential transparency? Referential transparency definitely has its victims, and debugging is one of them. Debug.Trace is quite useful, and also violates referential transparency. That Haskell provides it is an admission that strict R.T. is unworkable.

> If I'm writing pure code, then order of execution is irrelevant. It simply does not matter. If I'm writing impure code, then I encode order of execution by writing imperative (looking) code using do-notation, and it looks and works just like it would in any imperative language..

Baloney! Haskell's laziness makes the order of execution highly counter-intuitive. Consider:

import Data.Time.Clock
main = do
start <- getCurrentTime
fact <- return $ product [1..50000]
end <- getCurrentTime
putStrLn $ "Computed product " ++ (show fact) ++
"in " ++ (show $ diffUTCTime end start) ++ " seconds"

This program appears to time a computation of 50000 factorial, but in fact it will always output some absurdly short time. This is because the true order of execution diverges greatly from what the program specifies in the do-notation. This has nothing to do with purity; it's a consequence of laziness.

> Turns out that laziness, immutability, and referential transparency really helped this particular case

I don't buy it. In particular, laziness is almost always a performance loss, which is why a big part of optimizing Haskell programs is defeating laziness by inserting strictness annotations.

> Laziness meant that a naively written algorithm was able to stream the data from disk and process it concurrently without blocking

This would seem to imply that Haskell will "read ahead" from a file. Haskell does not do that.

> Immutability meant that the Haskell version could extract sections of the buffer for processing just by returning a new ByteString pointer. Other versions needed to copy the entire section into a new buffer

Haskell returns a new pointer to a buffer, while other versions need to copy into a new buffer? This is nonsense.

Like laziness, immutability is almost always a performance loss. This is why ghc attempts to extract mutable values from immutable expressions, e.g. transform a recursive algorithm into an iterative algorithm that modifies an accumulator. This is also why tail recursive functions are faster than non-tail-recursive functions!

> Referential transparency meant that we could trivially run this over multiple cores without additional work

It is not especially difficult to write a referentially transparent function in C. Haskell gives you more confidence that you have done it right, but that measures correctness, not performance.

Standard C knows nothing of threads, while Haskell has some nice tools to take advantage of multiple threads. So this is definitely a point for Haskell, compared to standard C. But introduce any modern threading support (like GCD, Intel's TBB, etc.), and then the comparison would have been more even.

When it comes to parallelization, it's all about tuning. Haskell gets you part of the way there, but you need more control to achieve the maximum performance that your hardware is capable of. In that sense, Haskell is something like Matlab: a powerful prototyping tool, but you'll run into its limits.