I've owned the Frontier for over 8 months now, and I am selling it today. Any major problems? No, just a coolant leak. Maybe I am too young and immature yet, but the truck, while not bad in any particular way, is just not what I was looking for. I would keep it if perhaps if I needed a truck and wasn't using it as a daily driver.
Bed extender, and the truck can actually hold quite a bit of weight. Get the 5 speed, I can't imagine how slow the automatic is. Reliable truck.
Gas mileage averaged 23 mpg with my sensitive driving and flowmaster. I've calculated it up to 26 once if I tried to hypermile - but I'd spend all day from point a to b. No cc and I think my biggest complaint was gas tank size. I couldn't get 300 miles on the highway with a 13 gallon tank.