That is not bad at all! My commute referenced above resulted in 28mpg - 75% hwy 25% city. Just highway driving at ~65mph, I made it over 420 miles on his car once, and got 35mpg. I filled up with just about 12 gals.
I kinda wish I had bought an earlier Vibe for myself, as I could easily go 2 weeks on a tank, but I really like the look of the newer GT. But the 24mpg I get is much better than the 17mpg I got on my Mustang.