I switched to a Q3 abouit 3 years ago from an Acratech. I use it with a RRS lever clamp QR plate. Never given me a spot of trouble - just got back from a week in Death Valley without incident. The drag mechanism works quite well. It's inset slightly, but is adjustable by finger. The big RRS BH 55 is heavier, and, from what other folks indicate, the Markins is smoother than the BH 40.
My largest lens is the Canon 300 2.8 and it's fine for that.