I used gravel in the plastic grids, particularly for the free draining properties. Both for a car use area, and for a pedestrian access path - ok for wheelchairs. I bought grids backed with terram by Nidagravel, which were a bit more expensive at c. £10 /m2 IIRC, but very easy to use. The irritation is that the gravel size specified for the grids is very small and drifts a little on boots and tyres. I am planning to try a slightly larger size of chukkies [14mm] to see how they ride, and if they bed into the grids sufficiently. Generally pleased with the system.