CI's email library has 3 modes: PHP Mail, Sendmail, or external SMTP server. In my experience, using Sendmail is very fast if you have a mail server installed on your webserver. I haven't tried PHPMailer because I haven't found CI's mail library to be missing anything that I needed.
The reason why we started using it was we are required to send out HTML email. This sucks I know, but it wasn't an option. I found that CI's email lib had problems with sending out HTML mail with images as attachments and referencing those images in the HTML. Absolute image urls were not an option either.
