I've run one of these for a while, which I use with the excellent Kawada pinion gears. I've had no problems at all with it - I only ordered a new one for my pit-box just in case becuase I didn't want to replace it with another spur gear from a different manufacturer.
I had not heard of Kawada before, but they have become a trusted brand for me - I will always be using Kawada gears from now on in all my cars, they are very well manufactured.
As I said, I have running Kawada gears in my TRF415MSXX which has all the gears exposed. I often don't run it in perfect track conditions and use the car outside my house, in car-parks and places where there can be a fair bit of dirt and grit - but I have had no problems what-so-ever with these gears so I'm sticking with them!