As a previous GTI owner, and a current 2011 Golf TDI owner, here's my take.
The mileage is good, especially trips. The torque response is good, if you drop it into sport mode, it feels much more like a gas engine with the extra boost.
But all the other things that make the GTI fun, are gone.
The Golf suspension sucks compared to the GTI, period. Everyone keeps saying the TDI has a sport suspension, what sport?? Certainly not motorsports. You cannot make a turn without the car scrubbing speed from all the chassis roll. In order to make fast turns, you cannot charge the turns at all, slow up early roll through and hit the throttle in the apex, but the car still scrubs. Also, you'll need take the inside line on turns, especially left, to minimize the body roll.
The cool steering wheel is gone, and the seats aren't as good. Definately feel the difference in the increased ride height, and thats not very good.
Oh, and make sure you keep the air pressure up at 40psi cold, or the conti's constantly feel like a bicycle riding over dried mud. There's a big mileage difference too if your air pressure isnt high enough.
To be fair, if you're coming from any other car but a GTI, you'll probably be impressed with a TDI. If you're a GTI owner, you're going to want to do a suspension transplant asap.
It's a pretty decent car to drive around southern california, freeways and streets. I love the bluetooth setup in the car, VW did a great job on that. Between a baby seat, the baby gear, golf clubs and the rest of my junk, I can still hit up target and go grocery shopping and have room for everything, it's a good family car, especially with the low cost of diesel right now. Also, unlike the 2.0T GTI, the TDI doesnt drink oil. I had to put so much oil in the GTI between oil changes, it was nuts.
The other big difference in percieved performance is that fact that the TDI does not "throw you back" in your seat like the GTI does. So it feels slower. However, if you're not paying attention, you'll be hitting 90 pretty quick on the freeway. The car accelerates from 50 to 100 very fast, it's just not the fastest 0-60mph car. Once the diesel gets a head of steam, watch out. It definately has passing power at any speed, especially on the freeway. However, it does not have the "light speed to endor" passing speed as the GTI. Sorry, went to disneyland this weekend.
I'm not bashing the TDI, it's a good car. I'm just giving an honest opinion of both vehicles. I'm currently driving VW#6, the wife's jetta is VW#7. The GTI was GTI #3. If you are interested in either car, borrow a buddy's for a weekend, a ride around the block from the dealership in either car tells you nothing.
MM48