StuffIt is a powerful and comprehensive archive manager that offers a number of additional features: it allows you to encrypt your archives, edit the contents of existing archives, create self-extracting archives, schedule backups and various compression tasks, and do many more things.
In addition to the proprietary StuffIt SITX format, it supports the already established archive formats, such as ZIP and RAR. This said, the SITX format has a lot of advantages, including better compression levels (which means smaller archives), so you may want to consider using it as the default format for your archives.
When it comes to downsides, I think the interface could use some improvement: it definitely needs to be a bit simpler, despite it already includes wizards that attempts to simplify the basic archive management tasks (creation and extraction). As it is, the interface may look a bit overwhelming to those who don't care about advanced options or compression parameters and just want to quickly extract or create archives. Still, StuffIt is a great tool despite that and its compression possibilities are unmatched by other applications.