I don't think learning XSLT made me a better programmer, but it is an amazingly useful tool for dealing with large amounts of XML (which is what it was designed for). That said, the way XPATH works is -- to put it mildly -- counter-intuitive.
My biggest gripe with XSLT is debugging. It's just not very much fun. Insofar as it forces you to reason very carefully about your code, that's probably good practice if you haven't already had plenty of practice -- e.g. developing websites before we had decent browser consoles, or writing INITs and TSRs in the 80s.
We were probably using it for very different things (I used it to render my HTML for a PHP site), but I actually found the debugging part to be a huge advantage of xslt. The errors I had relating to basic config and encoding issues were all big headaches, sure, but debugging issues caused by my own incorrect assumptions about how my data was structured was great -- I never felt lost and every message gave me a clear idea what I needed to look at.
My biggest gripe with XSLT is debugging. It's just not very much fun. Insofar as it forces you to reason very carefully about your code, that's probably good practice if you haven't already had plenty of practice -- e.g. developing websites before we had decent browser consoles, or writing INITs and TSRs in the 80s.