Mason is a very powerful system, it all depends on your needs. If your target is to have a perlish replacement for PHP, then I'd fully suggest it, having had to work in both a great deal.
My preference is strongly on the perl side, so take this with the grain of salt it's worth, but I find mason to be massively more flexable than PHP.
As for a general purpous templating system, both cephas and merlyn have good points. It comes down to that, you can do nearly anything in Mason that you can do in perl, with the display code nicely seperated, but still closely associated. This is good, but you have to be careful of adding in to lengthy of code into any mason component. Nutshell: The more perl there is, the harder it is for an html design person to come in and edit anything. You have to decide for yourself if that's what you want.