We've only had the cover 2 weeks but love it!!  Plus, the kids love the trampoline even more than they did before because the cover gives it a bouncy castle feel.
Our trampoline is under trees. Because of this, the amount of bird poo, leaves and tree sap that was always on the trampoline was annoying.  Plus, trampolines get very hot in the sun due to the large black jumping surface, now it stays nice and cool!