Yes, definitely, absolutely, worth the money. We've been 3 times over the last 6 years. First time DD and DN swam with the dolphins and DH and I just watched. Went back a couple of years later and no one swam with the dolphins, as we just snorkled and explored the bird avary (spell?) and had a great, relaxing day. We just took my DGD in November and she my DD swam with the dolphins. Each time was a wonderful day.
Yes, it is very expensive but if you utilize the admission to Busch Gardens or Sea World, it makes it a better value. Also, everything is included from the second you get on property (with the exception of photos and soveniers), including (this time but not our first couple of visits) the beer. They also have a continental breakfast and lunch and snacks.
You'll find more info on the Discovery Cove board, but for us it's been a great experience.
Jenn