I got my RB Pro-S a couple of months ago, and soon after got the Mamiya Multi-Angle Grip for it. I think the same grip works on the RB67 and the C330. This grip push-button adjusts to whatever angle is most comfortable, which comes in handy if you're switching between finders. There's an oversized shutter trigger on the grip, a wide soft leather strap, and a lock which locks the shutter from the trigger, but does nothing to lock the shutter button on the body, and since it's got a larger profile with the grip on it, you may still accidentally release the shutter with the grip on.
Originally Posted by snegron
I've heard some grips don't have a tripod mount, this one does. One disappointment, though, is that with the grip on, you can't use a cable release.
Of course, if as if the RB wasn't big enough, the Multi-Angle Grip makes it even larger, but it does make it a great deal easier to use handheld.