DSDs are great, and I've taken some people on DSDs who ended up loving it so much they are now Instructors.
Caveat - make sure, regardless of agency, that if there is an "Open Water" session, that the shop adheres to all required standards regarding skills taught and maximum depths. There are a lot of places that offer shallow water intro dives and this is fine, but not as complete, and I have personally witnessed these intro dives taking place in locations where I worry about experienced, certified divers, never mind newbies. As a previous poster mentioned, I've seen nervous people overwhelmed with the magic of the underwater world, and I have seen grown men cry when they dipped their heads underwater. Seriously. It's not for everybody.
It's a marketing tool, and it's a good one. It's a relatively inexpensive introduction to the underwater world, even if it's in a pool, helps people to overcome basic fears without having to invest in books or classes or whatever. For a lot of people, it's a box to tick, for some it's the gateway to diving. There is, of course, a great deal of cynicism surrounding the whole thing as a "money making tool", which of course it is, but conducted correctly, by the right person, they are a very good idea indeed.
Cheers
C.