At the moment, it's actually cheaper to use the Superferry (when it's running) than to fly and rent, PROVIDING you're staying three or more days.
Once they jack the prices back up and put the fuel surcharge back in place (scheduled for sometime in the summer), that will turn into a minimum of 5-6 day stay for any sort of value.
Of course, for those of us on Maui, it still makes sense to use the Superferry for shopping trips beyond what you can find on-island, as long as it will fit in the back of a pickup.
They're really going to be catering to a specialized market -- not the mainstream that they're promoting to.
Oh -- for the record -- I've ridden it on two round-trip voyages. Once in early December, once in early January. The trip back in December was rough. The rest of the three were great!
One tip: don't pay extra for the front lounge. Once the trip starts, they stop checking passes for entry.