I've read and heard that nginx can cache dynamic content and make it static so it loads faster taking more of a load of apache. has anyone done this before? is it worth it? any pros/cons to it?
Your question is somewhat ambiguous, I'll take a guess and assume you mean using nginx as a reverse proxy.
The main benefit of doing this setup, is reduced load. Nginx would optionally serve all static files, and reverse proxy all dynamic generated content. There are major performance benefits to this type of setup, and it can handle more connections. The major drawback is system complexity, you now have two web servers instead of one. It also is considered a more difficult system to administrate. regardless, I believe this is a great setup.
I originally looked at this setup, and then realized I could do everything just from nginx. Once you break everything down, only a very few things are Apache only. You can even set up a static cache all from the one nginx server, less complexity.
I hope I could answer your question.
They could be talking about 'proxy_store' -- http://wiki.nginx.org/HttpProxyModule#proxy_store