Buddy has them on his boat. They are massive. I've got four of the RH5710 ones on my boat and they have been great. Only real advantage vs the 5710 is that the rod is more stable when you are cranking away in the holder, especially if you get their reel clamp accessory.
You can reel just fine with the rod in the 5710, but sometimes (depending on the reel), the rod/reel won't sit perfectly square in the holder. One thing I prefer about the 5710 is that you can tilt it all the way up which locks the rod in vertically and you can run at speed like this with the rod secure. The 6650 has a max tilt of 45 degrees.
In short, I would only get them if you plan to do a lot of fishing (and reeling) with the rod in the holder and want the extra stability.