It's been great for quickly putting together something simple and functional. It's reasonably fast if used with mod_perl or the like. And supports persistent database connections etc (essentially in any page you specify something to run once per initialisation, and then the rest of the page/code to run on every render). The autohandler support (a page or code that runs every time any page in a directory is loaded) is useful for setting up things such as sessions, or header/footers.

The downsides are that it's difficult (and I've never gone to the effort of discovering exactly how) to run a debugger on a broken page. I've always reconciled myself to merely use the Perl warnings and manually inserted tracing (print statements) to keep track of what is actually going wrong.

A lot of people don't like Mason because there is very little separation of code/presentation. However the truth is that Template Toolkit muddies these waters too.

On a side note I have been able to get Mason working on a large virtual web hosting provider in the USA - where they only provide CGI support. Because they use large powerful servers and throw a stack of accounts on them the "slow" CGI response is not actually that slow.