Yup, it depend alot on how you drive. also if its an auto or manual, autos get about 5mpg less. and the year, the new 05.5's get less.
I have a 2000 Jetta manual with an Upsolute stageI chip. I don't drive conservative at all with probably 50/50 highway city driving. I average 43-44 in the summer and 39-40 in the winter. the lowest I think I have got is 36 in the winter. And rarely get below 40 in the summer.
They actually can easily achieve the EPA rated mpg. 50 mpg highway is not that hard at all. there are many people over an www.tdiclub.com
that have got 60 mpg (now this is not easy to get, these people are nutts about mpg, but it is posible).
speed is one of the biggest things that can hurt milage. it drops off above 65 mph.
I will note that I just took a trip this weekend of a of total 280 miles. I was not trying for good milage. crusing anywhere from 65 -80 mph, lots of accelerating and slowing down. I got 46.5 mpg. I was pleased with this. If I would have crused steady at 55mph I would have got over 50 mpg.
If you are looking for good milage I would highly recomend a manual 5sp.
When buying ask when the timing belt was done and if all the rollers and water pump were replaced. also ask if the intake manifold has been cleaned.