I own at the Marriott Ko Olina, and last summer I was literally almost run off the road on H1 by a Speedishuttle driver!
Sorry, had to throw that out there.
I've heard they're good, though.
We always have a rental car the whole time. We run out daily for shopping at Target, Costco, or to the flea market, etc... (oh and our favorite sushi spot in Kapolei).
We used to live on Oahu so drive up to Kailua at least once a trip.