.5 I really think you should have moved the banner to the back. How can he see where to steer that thing? The only other thing is the stance. If you look at the profile shots, the beast appears to be leaning back on it's legs a bit. Normally beasts appear to be leaning forward slightly.
This is very cool. Though I think a little post cure carving would have really sharpened the edges on the beasts armour. I might have had the banner behind the rider as well as all he has to look at is the back of his own banner. Of course an oger probly has ogress pin-ups glues back there with gnoblar spit. :) Also as more of a personal note it would be cool to see a pic of this unprimed, to see just what you did to sculpt the monster.
An impresive volume of work, however, the Rhinox looks a bit chunky on the armor bits, especially the head. Aside from the Rhinox head you balanced the elements pretty well. Really more of a 6.5 than a true 6.