I took the SuperFerry in December to Maui. It was a very rough ride and took a long time to get there.
What I didn't expect is how many people would get seasick. If you are susceptible to seasickness, I recommend taking Bonine or Dramamine or something. LOTS of people got sick.
It was extremely nice having my own car on Maui. You don't get that bad perfume smell covering up smoke.
I would say that the SuperFerry is only for locals. If you are a tourist, I wouldn't do it. You don't save any money on fares, it takes a lot longer and the schedule is not reliable. If the waves and weather are too bad, they cancel the voyage. If you don't have accommodations on Oahu, you'll end up getting a hotel room while your nice timeshare on Maui goes empty.
If you go for 2 weeks or more with your own car, it's a great deal. For anything less, I'd just take a plane and rent a car. You'll save, time, money and aggravation.
I don't think the SuperFerry will servive its first year.