I used Mail::Sendmail and a few others "older" modules back from the days when it didn't support setting envelope sender different from RFC From, and when the test hung on some dead host.
If it's still working for you, great. I personally have moved on to other modules like Email::Sender::Simple, which abstracts sending mechanism (transport) and support SMTP auth, for two. Also, many of the guide/documentation for Mail::Sendmail are not quite up to date in style (though they still might work), for example the low level way of building HTML email. Also, the Changelog file doesn't seem to be maintained?
It's all about the interface! While it isn't perfect, or even phenomenal, it is phenomenally easy. I can drop in quick email notices wherever I want, without hassle, and it just works on Win32, GNU/Linux, and Mac OS. Excellent.
I've used this in a number of different applications now, from little deamons to a large LIMS system. I've never tried to do anything complicated with it, but for something simple to use to send email notifications, text reports, etc. I've found this to abe a fantastic tool. Fairly portable, easy to install and use. I have yet to have encountered a single problem.
There are other modules for sending SMTP mail (Net::SMTP and the Mimetools spring to mind), but this one is my favourite. It has a very natural interface: a hash. Each key corresponds to a header (and you can add/invent whatever you like) and the "body" corresponds to the body of the message. Call the "sendmail" function with this hash, and off goes your message. Simple as pi.
Mail::Sendmail is an easy to use, platform independent mailer. I've used it on both Win32 and Linux. Although it has sendmail in the name, it does not require (the command-line) sendmail to function. It has a few short-comings (no SMTP AUTH support, for example) but it gets the job done. It also comes with a handy email address validation regex. The off-site FAQ has some useful example code for things like HTML mail and attachments.