Just to explain a bit more about why people are unanimously (and correctly) saying that PDK is so much better than Tiptronic...
Triptronic is a traditional automatic transmission. That means there is a fluidic coupling between what your engine is doing and what the drive shaft to your wheels is doing. When you punch the throttle, the engine has to "spin up" a bit to drag the fluidic coupling along, to make the wheels turn. Also, gear changes (no matter if the car is doing them for you, or you're using the steering wheel buttons) happen, without going into too much detail, in a fluidic sort of way.
Overall this means that your control over the power applied to the wheels is "squishy" and lags your input, and gear changes cannot happen very quickly.
PDK is completely different. Rather than a fluidic link, the coupling is through a true, hard connection via a clutch, just like in a manual transmission. The difference form a manual transmission however is that rather than having a single clutch plate (the two surfaces that are pressed together to join the engine to the drive shaft) that you have to control with the third pedal, there are 2 clutch plates that the car's computer can quickly go back and forth between.
The result is twofold. First, the power to the wheels is very precisely controlled by the accelerator pedal, just as in a manual transmission car. Second, gear shifts are nearly instantaneous -- *much* faster than can be accomplished in a traditional automatic, and in fact faster than almost any human can push the third pedal, change the gear, and release the pedal again.
Fundamentally, PDK gives all of the benefits of Tiptronic while also giving better responsiveness and control than Tiptronic and quickness of shifting that's better than either Tiptronic or a manual. The only reason to buy a manual is to either save money or because the driver prefers the fun of shifting themselves (which is why I chose a 6-speed). Tiptronic was always a bit of a compromise for the sake of automatic shifting -- PDK is the "ultimate" transmission available.