I had an '05 Aveo, 5 speed manual base model. It was definitely not the highest quality vehicle, and thats putting it nicely. I prefer my '99 Metro. However, mine was horribly beaten by what seemed to be a single mom and a messy kid (found so much junk in that car). She eventually sold it when she snapped the timing belt and put a hole in the block. I got it cheap and resold it after dropping a new engine in and running it for half a year. During that time I averaged mid 30s to lower 40s with real mild ecodriving, and a fair amount of highway (55 mph). My fuel log is here:
http://ecomodder.com/forum/em-fuel-l...vehicleid=3955
That being said, they are cheap cars that get okay mileage. There are certain things that tend to wear out on them, but its really not that bad if you know how to do a little car work. Darin put together an great list of things to look for when buying a used one here:
What to watch out for when buying a used Aveo