Any 150cc (automatic) will do the trick. The quality varies by brand, of course, but the 150cc 4 strokes are good for a top end of 60-65 and can easily hang with urban traffic.
One of the import (Chinese/Taiwanese) brands that has good parts and online support is Vento. Vento Yahoo Forum
Kymco makes an excellent 250-class scooter called the Bet & Win. It's cheaper than the Yamaha/Honda/Suzuki offerings and has a 2 year warranty. I have a Kymco Super 9 and it's gem-like in quality. I like it more than their Grand Vista (and other maxi scoots) because it keeps the shorter wheelbase that allows you to sneak it into any parking place, vs. being relegated to standard parking that the maxi-scooters need.
If you are looking for something that's more than just economical transportation--i.e., you want some retro appeal, long-term durabilty, and the ability to do some hot-rodding, check out the Stella
which is a LML-built Vespa PX series with a 150cc 2-stroke reed valve engine and 4 speed transmission. If you have any gear-head leanings and want something that's a genuine piece of history that evolved into one hell of a commuter that can actually do some touring, it's the way to go. $2900 new, about $2200-$2500 used. There's a good deal on one in Macon GA for $1900.
For a combo of the two--4 stroke 150cc bulletproof motor 4spd tranny and PX-series Vespa appearance, the Bajaj Chetak is a winner.
If you are looking to get into this cheap, look for the old Honda Helix and Honda Elite 250.
Money no object? Big Ruckus.
For cheapo transportation, try the Vento Phantom (150cc). I think it would be a one-year purchase, however--you'll want to move up or around after trying this. I started out with a 50cc Vento to see if I liked scootering, and went from there.
For the cost-of-entry and long-term useage I'd recommend the Stella, Chetak, or used Elite. That way you recoup your purchase price fairly quickly with the gas savings.
If you're doing a lot of freeway travel, I'd look at the 250 class 4-strokes and the maxi-scoots (250/400/500/650cc). Check with Ilmostro on his commuting experiences on a 500cc Aprilia.