I'm clearly of the minority opinion here, but personally I don't think it's worth doing unless you have a strong desire to actually touch the stingrays (or maybe have kids with you?). I've done it several times on both scuba and snorkeling. Your chances of seeing Southern Stingrays on other dives in Grand Cayman is quite good (I've seen as many as six on one dive there), so it just doesn't seem worth it to me to go with the hoards to see them. That said, if I were to do it again, I'd snorkel. The dive itself gets pretty repetitive after about 10 minutes as another poster stated. After the stingrays move on, the dive master will then lead you around a few coral heads but there is just SO much sand that visibility to pretty poor.
BUT, if it actually seems intriguing to you and you might regret not doing it, then by all means check it out. I don't think it's terrible or anything, would just prefer spending my time there doing other dives.
I'm sure you will have fun whatever you decide!