I was driving my DF-03Ra in dirt a lot and the original joints collected a lot of dirt. I had applied grease on them as instructed in the building instructions and all the sand just stuck in to greasy the rear joint.
The metal joints are connected to the shaft with plastic pieces that make it fit perfectly. Because the fit seems very good and the shaft rotates very well with the new joints I haven't applied any grease to it at all. This makes the joints much easier to keep clean. Also, with the tight fit and without the dirt these joints make the shaft lighter to rotate for the motor.
These joints look like they only add bling bling to the car, but having them, I think I got good value for my money. I'm a little worried about adding too many metal parts to my drivetrain and therefore making the gearboxes the weakest point on it. To avoid that I'm getting a slipper clutch set.
For me this is an essential upgrade - especially if you have upgraded the drive shaft to a blue one (from the stock silver metal drive shaft). As an added bonus you also get two driveshaft protectors that clip onto your existing drift shaft to stop noise and rattling. As a little build tip, I also put one of the small rubber 'o' rings into the end of each - this seems to give a perfect fit with just a tiny little bit of movement to stop it rattling around.