I've got a 2004 Jetta TDi with a 5-speed. I commute 120 miles /day (and is the reason I sold my 4Runner). 85% of the miles are highway. My fuel economy is roughly btw 49 and 53mpg. The car is stock. I have approximately a 700 mile range if I'm not conservative (that's with a 75mph cruise).
I didn't want to buy it, honestly as I loved my 4WD. I just couldn't justify the fuel price. I considered a hybrid, but I do mainly highway driving. Once I drove a TDi, I was hooked.
There are many things you can do with the TDi. In 2004, the engine was changed to a PD style (directed injection/more ECU controls...). TDi engines prior to '04 are easier to work on, and have more available aftermarket bits. However, the new PD style engines have higher base HP, and aftermarket parts are becoming more available. Chips, injectors, etc... You can increase HP and torque, while improving mpg.
I have so far put on 20K miles on mine (bought with 28K on the clock). Most important with any TDI is the oil used in the oil changes. Due to very high internal tollerances, and the inherent nature of the high compression, you must use only the VW recommended grades. On the plus side, oil change intervals are at 10K miles.
I enjoy driving the car immensly. It takes about 10 mins to get used to driving a car with torque vs HP. The turbo is quick to spool. Engine is very quiet. Car is very well built overall.
The best place for more information on the VW diesels is a forum (much like ih8mud). It is the
www.tdiclub.com. Classifieds, tech, etc.
Feel free to PM with any specific questions.
uberhhan
(you can also find me on tdiclub under the same name)