For a long, long time, XML-Spy has been the dominant (and only) XML/XSL/X-Whatever tool we used in Tridion.
Since our product is extremely heavy on XML and XSL, this is an invaluable tool, and all of us use it - despite its rather steep licensing price.
Then, one day, I needed to debug some XSLT that was not transforming correctly using Xalan. It did work perfectly in MSXML and the internal built-in XML-Spy processor.
Cutting a long story short, I ended up downloading and trying <oxygen/>, and simply fell in love with it. So, I asked our internal IT department to get me a licence.
Up to here nothing unusual. Except for the little fact that Oxygen's licences can cost about 25% of XML Spy's. So, IT is now trying to convince the whole of Tridion to use Oxygen instead. I don't really care which way it goes, as long as I get my Oxygen licence...