I have bought the DFGT last week. Here is my personal experience with it and the GT series in general. I will try to make it short, please bare with me

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GT1 - Barely played it, did not like it at all at the time, shame on me, I was not too much into simulators. To be fair, I have not given it a good chance back then.
GT2 - GT mania started to hit me there, and I loved that game. Not sure what changed, but I started liking the more realistic feel of that game as opposed to the traditional Need for Speed arcadish games I was used to and loved. I played the whole game with the DS1 controller and got around 85% completion on it.
GT3 - Got a severe case of the GT fever, and it completely took over my life for about 6 months... I mean completely. It is all I was doing besides working, eating and sleeping. Got 100% completion on that one. I played about 25% with the DS2 controller, and then got myself a Logitech GT Force wheel and completed the game with it never going back to DS2. I thought it made the game much more challenging and interactive, and easily more fun and engaging. At first I sucked bad, but after a few hours, if I remember well, I started to get a good feel for it, and soon became a good driver I like to think. Note here that the GT Force had only 180 or 200 degrees rotation, but it felt right, especially with the F1 and Lemans prototype cars. F1 races brought my adrenaline levels to new grounds, and I was completely addicted

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GT4 - I guess my immune system had kicked in and was working overtime with this one, cause I definitely did not catch the fever as strong this time around. Only completed 60% of this game, and didn't even bother setting up my wheel out of laziness. Still loved the game though and I think it was a continuation of the GT quality we have gotten used to, I was just not willing to let it take over my life once again. I won...
GT5P - Loved it, completed 100%, not much else to say, game too short. Played mostly with DS3, some with GT Force, but I did not have a good setup since I had moved. Let's move on.
GT5 - DAMN!! DAMN DAMN!!! A 6 years + wait really works wonders making your immune system weak I tell you. The fever is back with a vengeance, and has me pinned down, completely deprived of a social life, or a life altogether. GT5 is all I do, besides working and eating. Notice how I left out sleeping... not good

I am sooooo loving this game. 95% completion so far, have not even scratched online yet... my life is over!
I know this story has not been about the DFGT so far, but here comes the meat... and sorry for sidestepping from the topic, but I had to share

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With GT5 I started off with the DS3 because I did not have a proper setup (cockpit). About 20% of the way, I switched to my trusty GT Force once again, since I made myself a decent cockpit and I am now properly setup. WOW... again the wheel made all the difference in fun factor and player interaction. Then the pedals started acting up... the throttle would let go in the middle of a straight while accelerating hard, without me letting go of the gas pedal, or car just dies coming out of a curve when the pedal does not register me stepping on it. Long story short, I got pissed, and went out to replace it with a shinny new DFGT. Was I happy

! This was about 90% through the game.
OK my DFGT impression so far... I like the 900 degree steering, makes the experience even more realistic in my opinion. However I have to say it feels a little weird when driving an F1 or a prototype race car, it felt a lot better and natural with my old GT Force at 180 degrees. But this is true only for those cars, and it is still manageable with the DFGT, it just requires a little more skill and practice I guess. Every other car feels better and smoother with 900 degree rotation I must say. The FFB is awesome. It is stronger than on the GT Force, and the feeling it gives seems more lifelike. One gripe I have with DFGT though is when the car over steers and starts sliding it is really difficult ( for me anyways) to recover, and 90% of the time, my cars snaps the other way during the counter-steer and it is usually wall kissing time or play time in the sandpit

! This is the only thing that makes me miss my old GT Force sometimes, as I rarely spun out with it, and was always (almost) able to recover from over-steer sliding. Maybe I will get better with more practice, I have only had my DFGT for a little over a week as mentioned previously.
All in all, I am loving GT5, and DFGT makes the experience so much more entertaining... to think of any fan of the series who doesn't get to try this is really sad. Those who can afford the wheel, and hesitate to get one, just take the dive, I guarantee you will not regret it.
I appologize for this lenghty post and I hope I didn't bore anyone to sleep, and that my feedback can help some players make their decision.
And last, I just wanna share my baby with you guys, I just love it too much! - Ferrari 458 Italia fully tuned Bling Custom from my Chrome Fleet... feel free to comment

. Don't worry, yes I do have a stock Italia as well.
S_D out!