I'm surprised this topic has this much interest
I have an Aprilia Scarabeo 500ie italian scooter. It currently has 18,000 miles on it, and I haven't done anything to it except basic maintenance. Top speed is 99mph, and it gets 65-70mpg. I paid $3k for the thing. I do my freeway commutes on it. It's a really underrated scooter, and the Aprilia/Piaggio 500cc motor is very well understood and supported. This is the "light" version -- the older Scarabeos (pre-2007) were tanks, made for touring. Those owners love theirs, I love mine. Two different purposes though.
We also have a Yamaha Vino 125 in the family, which just turned 10,000mi. I use it for city only since it tops at about 55mph, and it's perfect for Los Angeles -- it's a lane scalpel, and I only stop for red lights, otherwise I'm a lane-splitting fool -- commutes that are 45mn by car are 10-15mn by Vino. It gets 90mpg. Same deal, haven't done anything but routine maintenance on it.
My dad has a Buddy 125, and it's a nice scooter. Too new to speak to longevity, but it feels very sporty for its engine size. That leads me to believe that it will "burn brighter, not last longer", but we'll see.
I've eyed the Honda PCXs covetously, since they're fuel injected and high compression motors turning in some very high gas mileages (I think the PCX 110 was over 100mpg). They'd be on my short list if I was in the market for a 125cc-class scoot.
250cc is an odd size and there don't seem to be many players there. Honda made a 250cc ruckus which is styled like an army jeep, but I bet it's a blast with that engine. Aprilia used to have the Sportcity 250, but I didn't care for its ergonomics (I'm big and tall, and that scooter was made for the waify italian college girls apparently -- way too short for me). There are also a few 300cc sized Vespas, but the Vespas tend to be very highly priced for what they are.
Everyone I talk to says the korean/chinese scooters are junk, including my scooter mech. I haven't bothered with them, so I have no idea. I do see the Kymcos more often than any of the other brands, but I'm not convinced they're going to go the distance yet.
$0.02