I picked up a pair (you get 2 in each pack) of these to replace a damaged one in my Tamiya TRF415MSXX (YES, they also fit an MSXX even though in the description it states it will fit a 415MSX).
As always with Tamiya, the construction quality is absolutely A1 perfect. They actually feel very light - lighter than the standard black ones that's for sure. They will improve drivetrain efficiency as there is less weight to rotate, so they really are a very good performance upgrade if you are after best performance out of your car.
They will fit both the front and rear, although I'm told they are not quite as tough as the standard black metal ones, so some people run these just in the back and not in the front. I personally have them fitted to front and rear for the best performance - they look very 'pro' too, and people in the know will see your chassis and be impressed.
Basically - if you want the very best - buy these (and as always - a good price from RC Mart!)