I just sold an old Honda Spree. (My cylinder count would have just went down by one - but I just picked up a tiller, so it stayed the same - WHEW!) It was just a little 48 or 49cc job. IIRC, it got ~85 mpg or so. Was fine for around town duty, but since it only did ~35 mph, it wouldn't be so great on faster roads. Was great for around town, but since I don't live in town anymore, it was time for it to go. The rules in WA classify it in the same category as a moped (<50cc, top speed 35 or less), so you don't have to have a m/c endorsement to ride it. The wife loved bopping around town on it. I rode it a few times, but I'm actually heavier than what it was rated to carry.
I think the newer, bigger scooters, like the Bergman & SilverWing are great and would be good commuters or even long distance machines. I've heard of at least one being ridden in Iron Butt events and doing well, so there doesn't seem to be a question that they can't be ridden for LD work.
This new breed of scooters look so much like a motorcycle from the front, it's often hard to tell if they're not real motorcycles. I'd wave to 'em even if I knew they were scooters - but then I wave at scooters anyway... they're out there enjoying the 2-wheeled experiience too.
If I was trying to save every penny I could on gas though, I'd probably just buy the most fuel effecient motorcycle I could find that would fit the requirements of the commute - as long as it wasn't a cruiser. Though I must admit the cargo capacity of the modern scooters does have its appeal.