I've used Mason in a corporate environment where it was the main web framework for internal staff tools.

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.