Joeyh: "One (git-annex) is a large-ish, serious work, and I have been very pleased with how haskell has made it better, even though there was a learning curve (took me two weeks to write the first prototype, which I could have probably dashed off in perl in two days), and even though I have occasionally been blocked by the type system or something and had to do more work.

One concrete thing I've noticed is that this is the only program where I have listed every single bug I fixed in the changelog -- because there have been so few, it's really a notable change to fix one!"