Comparing
Struts to
JSTL is imho comparing apples to oranges. Struts is a MVC framework to use in building web based apps, whereas the JSTL simply encapsulates some functionality common to most JSP apps. Used together, you would build your application in a Struts framework, and use JSTL to assist with view logic in your JSPs. Struts also has its own set of taglibs, some of which duplicate functionality found in the JSTL. Here's a pretty good
article to help get you started in JSTL.
But to answer your question more directly, since JSTL is only a set of taglibs, there is imho less of a learning curve than there is with Struts (although as I stated the two really shouldn't be compared).