It was written just to solve my own problem, but in a general way, that turned out to be useful in lots of other situations. So over the first half a year or so, it started attracting some early adopters who made some very helpful suggestions.
Then I did the git-annex assistant kickstarter, and started blogging about each day I worked on it. Four years of funding and seven hundred and twenty one posts later, the git-annex devblog is still going. So, I won't talk about technical details in this post, they've all been covered.
One thing I wondered when starting git-annex -- besides whether I would be able to write it in Haskell at all -- was would that prevent it from getting many patches. I count roughly 65 "thanks" messages in the changelog, so it gets perhaps one patch contributed per month. It's hard to say if that's a lot or a little.
Part of git-annex is supporting various cloud storage systems via "special remotes". Of those not written by me, only 1 was contributed in Haskell. Compare with 13 that use the plugin system that lets other programming languages be used.
The other question about using Haskell is, did it make git-annex a better program. I think it did. The strong type system did prevent plenty of bugs, although there have still been some real howlers. The code is still not taking full advantage of the power of Haskell's type system, on the other hand it uses many Haskell libraries that do leverage the type system more. I've done more small and large refactorings of git-annex than on any other program I've written, because the strong types and referential transparency makes refactoring easier and safer in Haskell.
And the code has turned out to be much more flexible, for all its static types, than the kind of code I was writing before. Examples include building the git-annex assistant, which uses the rest of git-annex as a library, and making git-annex run actions concurrently, thanks to there being no global variables to complicate things (and excellent support for concurrency and parallelism in Haskell).
So: Glad I wrote it, glad I used Haskell for it, estatic that many other people have found it useful, and astounded that I've been funded to work on it for four years.