I recently bought a "clip on" Yagi-Uda antenna with the following results in a relatively clear field:
1) range with stock controller/antenna ~7500 ft
2) range with Yagi extender ~10,500 ft
So a decent increase in range for a small cost (~$10). Remember that range is still dependent on surroundings; e.g. lots of trees/buildings will significantly reduce the effectiveness of these Yagi extenders. However, you would have to spend a lot more $ to get a lot more range...and at that point you're flying beyond VLOS anyway. Needless to say: I'm happy with my purchase!