OK. I am biased. I love Paul Wirt pens. With that out of the way, let me tell you that the Wirt hump filling pen is the most elegant of all. (I might let it be a tie with the Grieshaber hump filler.) It is not the fastest to use. In fact, it is the slowest. But there is no “cheapness” to it, unlike many of the other hump fillers that seem rather basic. It is the only hump filling family member with a hard rubber hump, and it has only minimal exposed metal from the filling system lock. It is well engineered for smooth operation, unlike some locking rings that can get stuck or break. The hump is locked by a flat bar that runs inside the barrel and is connected to a knob at the end of the barrel. Unscrewing the knob results in pulling back the locking bar, allowing the hump to be used. It takes 5 or 6 twists to open, and to lock again.