I dove with Mauna Lani Sea Adventures last year and plan on diving with them again in mid-April on my honeymoon. My expectations for them last year were pretty low (since they were on site operator and I tend to do advanced diving), but I thought they did a nice job. While my memory is fading a little, as I recall I think the staff I had 10+ years experience.
As I believe one of the other posts indicated, they generally do dives that were near the resort. The staff was friendly, organized, on-time and did a solid job. While we didn't do any deeper diving, they seemed pretty flexible to accomdating more experienced divers. A repeat guest wanted to do a dive that was primarily swimthroughs and I tagged along. I think we probably did 20+ swimthroughs on one dive, several of them were very tight (I am overhead certified, so I have seen my share of tight swimthroughs).
They did a resort course for my girlfriend (now fiancee) and they did a good job on that also.
I would not look to them to do any nitrox/deep air dives, but I wouldn't hesitate to recommend them for general diving. I believe they were pretty flexible in allowing me to dive to my air supply (60 minutes+).
I don't recall any manta dives up near Kohala. I dove with Jack's in Kona a few years ago and would use them again if you are just looking for a manta dive. They do a lot of volume, but were organized when I used them.
BTW If the food gets boring while at the Mauna Lani, Tommy Bahama's (In the Mauna Lani Shops) is pretty good. Not cheap, but good.