Its advantage over CCK Teaser and CCK Teaser Field is that the teaser is stored in the node teaser, which means that it works with other modules that work with the node teaser. Deck also falls back to generating a teaser from the node body, if the field is empty, using the fields maximum length as the maximum teaser length, falling back
to the setting of Administer > Content management > Post settings
Excerpt has the advantage of being able to set the filter used for the teaser (though this might be implemented in a later version of Deck), and not being dependent on CCK. However, being able to use the CCK field used for teaser as the deck (or subhead) when viewing the full node was one of the motivations behind this module.
As Drupal 7 has improved the teaser handling, I see no reason for further development of this module.
Developed at Peytz & Co.