I've got 4.10's.
They are very nice, but I think people over exaggerate their claims in performance.
I've seen claims of:
-Losing contol
-Inability to start off in 1st gear without spinning tires
-Relearning how to drive
-Throwing the car sideways all the way through 3rd
I suppose if I revved to 3k and dropped the clutch I would have these problems too.
Perhaps these people are only reporting in terms of racing at the track.
Personally, I just slowly come off the clutch and press the accelerator and I'm just fine.
I noticed a nice difference, but I didn't shiitt myself the first time I got on it.
Perhaps if I drove a stock GT again I would think differently.
One of if not the biggest exaggeration is the claim that they will "kill your gas mileage", most often said by those who have never had them. The change I most often notice with 4.10s is that they make fifth gear usable instead of the dog it was with the stock gears, and really woke up third and fourth gears. I never have any problems with spinning tires starting off in first.