The nicest thing about Hledger and the original[1] is the extremely simple file format. It allows you to write your own pretty easily (especially if you stick to the very simple format of a bunch of transactions). I wrote one for my use [2].
if you have used both, can you go over why someone might want to choose one over the other? I suppose since they share the same file format, it shouldn't be that hard to switch between them.
The basic file format is the same, but there are differences - with a little care you can keep a file that works with both. Abandon and beancount's file formats are more different.
I'm happy to answer questions here or on #hledger. I've been using and growing hledger steadily since 2007; I just released 0.24 and we are thinking about a 1.0 one of these months.
Contrary to what is said in this article, the GnuCash XML file can be checked into git, the diffs are quite readable and there are no additional files to track.
[1] http://ledger-cli.org/ [2] https://github.com/howeyc/ledger