If you think you want to do XSL/XSLT, then go get the book "XSLT Programmer's Reference" by Michael Kay. Great book! I work on a team of architects and we just finished up a two month study on the use of XSL.
The short answer is do NOT use XSLT on the web tier. In fact, don't use XSL for interactive presentation layers. XSL is directed at b2b data translations or document publishing translations.
Once you use XSL (side effect free) and then mix in to the source your XPath, XPointer, HTML and Javascript, your developers will suffer a productivity hit attempting to 'find' the code to update. Poor editor and debugging tool support to date. Developers need wyswig tools to go from prototype to production with as few steps as possible. Using XSL is counter productive in the complex interactive domain.
However, some hard core developers will need to know XSL going forward. It is popping up as a core technology in many areas of the XML family. 'Hard Core' developers would be those involved in frameworks and tool development. The average Joe application developer shouldn't need XSL.
Don't be swayed by the simple examples (unless you have simple applications). Also think about the web server performance hit on all of those DOM parsings. Here is a quote from the Cocoon Users Guide(
http://xml.apache.org/cocoon/guide.html)
'In a complex server environment like Cocoon, performance and memory usage are critical issues. Moreover, the processing requirement for both XML parsing, XSLT transformations, document processing and formatting are too heavy even for the lightest serving environment based on the fastest virtual machine.'
Good Luck
Scott