Bones has been perfecting urethane for forty years and it shows. If you take Rollerbones' 92a and compare a competitor's of the same size and hardness, one will find a noticeable grip advantage which converts to improvements in both roll and grip under your feet. It seems the majority of derby players have a set of these in their quiver and speak fondly of them.
Check out the 62mm Day of the Deads too.
Sold in 4-packs