I've used Mason on a couple of fairly large projects, and love it.
It's fairly quick to get up to speed on, uses perl, with a few special html-ish tags, includes a caching api built on Cache::Cache, and has quite a number of modules that extend the core system. (such as adding Apache::Session support)
I don't know enough about other systems, at least not any recent usage, but in my mind, the fact I don't have to learn a fancy mini-language for my templates, and just write them in perl, is a big plus.
Too many templating systems seem to have been written by people wanting to design a programming language, rather than a templating system.