I got mine yesterday. Have finished building the transmission (including slipper as I got one with the kit) and will start work on the suspension tonight.
The drivetrain is very smooth, including the diffs. No surprises here considering it uses all the new 501 pulleys. I did what was mentioned on the previous page and glue the diff rings to the plastic holder -the manual does actually tell you to do this. I also installed ceramic balls in the rear diff.
The car is also supplied with a 91t spur, I think that the original 501x 96t spur gear may be too big to fit (ie: rub on the chassis)
The drivetrain is not 100% sealed either. Front and rear cases are good but the central "tunnel" has a couple of small gaps - a bit of foam should sort this out. Also, there are two unused holes in the motor mount (for the optional belt tensioner kit) - they probably sould have included some plugs for these holes.
Speaking of plugs, the lower motor screw access hole is covered by just a piece of tape, again, Tamiya probably should've included a XX4 style rubber plug.
The shock towers were a big surprise as they are very stiff - not like the DF03 ones at all. The rear tower only flexes very slightly in the middle but otherwise both are very beefy and should even be good enough for competition use.
The use of seperate pieces for attaching the front and rear suspension is also a big improvement over the df03, and also allows front caster adjustments to be made - even the 501 didn't have this
The only dissapointing part is that tamiya have omited many of the small metal spacers used between bearings, inside diffs, ball studs etc and replaced them with less-precise plastic ones. I know they have to make it to a budget but I seriously doubt removing 10 or so metal spacers is going to make a big difference to the cost of the kit.
Otherwise the car all looks good, and to be honest with only a few "required" hop-ups I can see this car performing just as well on track as the 501x.