I have 2 complaints with my truck. It's averaging below 20 mpg. I previously owned a '99 S-10 V6 that averaged just over 20 mpg. I was really expecting the newer Frontier to be a little over 20, but I average about 18. Half of my 60 daily commute miles are highway. A few months ago I drove 1500 miles to visit family in California and it averaged 19 mpg (averaging 80 mph). I am very impressed with it's performance. Shortly after purchase the stereo won't play or eject CDs (err03), but it sounds great still. I am also considering buying a new extended cab Frontier when this one is paid off. Overall I'd give it about a 95% satisfactory rating.
Rockford Fosgate 6 disk CD with 7 speakers, bed extender, roof rack, power windows and doors,
6 spd manual transmission for improved economy (especially at highway speeds). Basically, keep the current ratios, just add a highway gear. Also, a stereo that allows you to manually eject CDs (but has the same quality sound!)