[DWPC]

Mileage: We got 17.9 averaging 70 mph with the A/C on from LA to Sedona and back (1200 miles) last weekend.
Cargo: I agree; cargo space is skimpy for a rig this size. That the rear seats don't fold flat is a PITA.
Front seats: I disagree and think H3 seats are way above average for long haul comfort.
Engine Power: I heartily agree. Between the I-5 engine and 4 sp auto trans, the H3 has a rough time keeping up with traffic on even slight grades without dropping to 3rd. It reminds me a lot of our old MB 300 diesel (non-turbo). The H3 is not well geared for 75 mph interstate cruising in the West where we have long grades and hills. A 6-sp trans would likely help this problem a lot without sacrificing economy.
Daily Driver: Our H3 is my wife's daily ride and I can't get it away from her. We're willing to accept the compromises because its road comfort and handling are really superb; especially for such a capable offroad beast. Power isn't everything.