Quite frankly, I think driving on a real track in a real car is actually EASIER than doing the same thing in GT4. That, of course, is assuming you have already driven on a road course, closed circuit track, or autocross course in real life, preferably a number of times. You cannot use GT4 alone to learn how to drive, however, because you'll probably develop habits that would not help you or be particularly safe to use in the real world, and you'll also be unfamiliar with the variety of sensory information so critical to a real world driver's technique and response that is not given to you through a 2D video screen.
In this sense, I think real world driving experience will help you more in GT4 (once you get used to the DFP) than vice versa.
Don't get me wrong, the basic techniques are the same. And I was absolutely thrilled when I first took the M3 (closest GT4 equivalent to my real world car) out for a spin on Laguna Seca in GT4 and felt the DFP respond like a real steering wheel/car would when losing traction on the front wheels.
I will say this, in regards GT4's physics engine: it is the best driving-style video game physics engine that I personally have experienced yet. The braking characteristics, tire physics, and ESPECIALLY weight transfer effects are getting remarkably close to mimicking the real thing. So in that sense, you can learn some driving techniques with GT4 and a wheel. I found that after playing GT4, my natural reaction in-game to losing drive wheel traction on a powerful RWD car (as a result of excessive throttle application) was the same as it was in real life, and in both cases the cars would respond the same way. It CAN be a useful learning tool if it's limitations are understood and the student does not develop bad habits through it.
However, you have a number of disadvantages in GT4 which do not exist in real life.
1.) As good as the DFP is, the controls still have a way to go. The lack of a clutch and H-pattern shifter are one obviously glaring flaw (the shifter controls on the DFP are more akin to something you would find in a car with a sequential gearbox - M3, M5, Ferrari 360, Toyota MR2, or most race cars - or even a shiftable automatic). The pedals have no feel or feedback whatsoever (granted, this is compared to my 3 series, which has a VERY tight throttle and VERY tight brakes...my other car, the daily driver Acura, has a loose disconnected feeling similar to that of the DFP), and the placement is too simplified to be realistic, at least if you were to attempt a heel-toe/ball-edge downshift (which isn't even necessary without a clutch pedal anyway).
The wheel itself is too small, though I do think the steering feel is nicely weighted and surprisingly responsive, especially for a relatively inexpensive $150-odd device. The force feedback is also very good, though it's sort of a compromise because most cars would never give you that sort of feedback through the wheel (though you would likely feel it from the chassis itself). It's fairly obvious that the bulk of Logitech's engineering work and production efforts went into the wheel mechanism itself, the pedals and shifter controls are mostly an afterthought.
There is one major problem, size aside: turning the wheel through it's full 900 degree motion usually feels very awkward and unrealistic. In both of my cars, the power steering effort is increased as the driver turns closer towards lock, to aid the driver in the application of rapid steering changes. This also helps the driver to know where the wheel's center point is by feel alone. With the DFP, this only seems to occur if you break traction entirely on the FRONT wheels. And the small diameter of the wheel makes it difficult to turn the wheel hand-over-hand as you would in real life.
The end result is that effective countersteering is very difficult to do, because you'll either overcompensate or undercompensate, and then returning to center is almost impossible to do by feel, either. This, I think, is where the majority of complaints are coming from re: how difficult it is to drift in GT4. Using the DSP controller, however, I had no problems getting cars to initate controllable four wheel slides that would do so normally in real life (provided I was using the more realistic N2/N3 tires - and, contrary to popular belief, very few real cars are easily driftable in stock form).
2.) You also lack the sensory feedback and depth perception that you have in a real car, which makes a tremendous difference in how you must judge braking distances and cornering speeds. In GT4, at first, I tried to brake as I would in real life: by looking ahead to the corner and it's apex (if visible). I rarely use the distance markers in real life. You simply cannot do this successfully in GT4 with any sort of consistency. Because the sense of speed is not the same (again, this is due ENTIRELY to depth perception) and because you cannot judge distance easily by staring into the 2D image on screen, the only way to learn braking distances in GT4 is to memorize them track by track OR use the distance markers on the side of the track. The same applies to cornering speeds.
Also, I can't feel the car sliding out from under me in GT4, so I must watch the speedo and memorize the visual feedback from the screen through each corner in order to make that turn consistently as I might in a real car. You wouldn't even think to do this in real life. The only other sensory cue to your state of traction in-game is tire squeal. Fortunate that all of GT4's tire models give early warning and have very similar break-away characteristics...which is not always the case with real tires.
If GT4 gives you, the driver, one critical advantage, it's that the lack of a sense of speed and danger will encourage you to push the limits more than you probably would in real life. You cannot wreck your $35,000 car in GT4 (even if you could, buying a new one, especially in the latter stages of the game, would be a small concern at best), nor can you die or be paralyzed from the waist down just because you wanted to improve your already-fast time through that 90 mph chicane.
