My Kia Spectra gets 35 miles to the gallon average. Since my purchase in June 2011, I have put 11,000 miles on my car, raising its total to 141,000. I have already replaced one tire, the engine damper, a shift cellanoid, two taillights and a headlight, and am currently in the process of replacing the trans relay switch. My transmission occasionally slips, and it doesn't have a "floaty" ride like some sites claim because the shocks quickly disintegrate. Overall, I do love my Kia despite severe hail damage and some minor issues. The cost of repairs thus far has cost me around 750$, less than half of what I paid for it. However, I likely wouldn't buy a newer model Kia because of poor reviews.
Definitely the gas mileage. I love to tell people that I get around 500 miles to the tank. I also love the roomy trunk. I sleep in my car occasionally between classes, and am usually very comfortable with the seats folded back. Also, the fact that the interior roof is not made of a loose cloth is very nice and easy to clean.
Also, the Grindstaff Kia dealership in Johnson City, Tennessee has always been really good about being friendly and informative to me, which I've heard can be a problem with some of the Kia dealerships. Maybe I just found the exception to the rule, however.
I've had several problems with the transmission, including occasional, but rare, slipping. I've also heard that some models, both 2002 and newer, have problems with the airbag deploying during a front-end collision. However, my mom drives a 2011 Kia Sportage and was shocked when her airbag deployed when she barely felt the collision, so it may just be certain cars.