I've done the dive twice and got no mantas once.
We went with Jack's both times. The time we got shut out (August or September '04), the DM for my group was Keller, who is a great guy, a manta expert and a great diver. The dive was a lot of fun even without the mantas.
When we went in May '05, we got 4 mantas...honestly if you get even one manta you'll have trouble holding in your regulator because your jaw will want to hit the floor. They are simply amazing creatures and they come unbelievably close to you.
Something I don't think has been mentioned is to wear more exposure protection than you normally would in Hawaii because if they show, it's a real couch potato dive. You just sit on the bottom watching mantas and wondering if that diver next to you notices the undulated moray crawling over his legs
Anyway, you do tend to get chilly sitting there, and it would be a bummer to call the dive because you're cold.
And wear more weight. I ended up grabbing rock off the bottom to hold me down really well so I could relax and check out the mantas without the surge shoving me around.
It was a great dive...truly exceptional to see mantas so close. Also consider a night shore dive at Two Step or elsewhere. I had an absolute blast going out at night independantly, and we had the whole place to ourselves.
I've also seen a couple of mantas during the day off the Big Island so always scan the blue water when you're diving, even in the day.
Have fun! I'm jealous. I absolutely love the Big Island.