Few points here:
- Love diving the Big Island (dive it every year), but Niihau is as good or better than on a really good dive on the Big Island. Diving Niihau (you actually dive Lehua Rock, just off of Niihau) you have a pretty good chance of seeing a Monk Seal (or two) underwater (if there is one around they usually come check out divers). It's also one of the best places to dive in Hawaii with sharks - good chance to see grey reefies and white tips, a few hammerheads, and the occasional Galapagos shark (and there was a unconfirmed sighting of a great white underwater several years ago).
- Underwater topography is unlike anywhere else you can dive in Hawaii. The underwater arches are amazing, and several of the walls are very deep. It feels very different than diving any of the other islands.
- More fish, more of the time... given the remote location Niihau hasn't suffered from fish collecting (or fishing) nearly as much as the other islands. You'll see more fish here than virtually anywhere else as well (although most of the dives are open to potentially strong currents, so these really aren't typical coral garden dives either).
One thing to keep in mind - you can find a number of reviews of snorkeling Lehua Rock where it's described as "deep and dark" and a little bit "spooky." That is actually a pretty good description of the feeling you have diving there - which is one of the attractions. It's still fairly wild - not great for snorkeling, but much better for diving.
I've been 4 times, and I'll head back again either this summer or next. Kauai diving is (at best) average, so it does make it hard to plan a dive trip around Niihau.
I'm assuming you know this - but during the summer they make the trip about 70%-80% of the time it's scheduled. During other times of the year it drops below 50% (too rough to make the trip). The trip out there is usually not that rough (actually pretty calm), but the trip back can be (and usually is) fairly "hellish." As an example, one way I've seen experienced divers ride it out is to leave their wetsuit on, put their mask on, put their snorkel in their mouth and lie down on the back deck (the whole way home). It's a multiple hour trip back, and usually very rough & wet (just the nature of that channel where your fighting the prevailing seas the whole way home).
- Matt