Nah, Kevin Martin isn't all that great at all. It's laughable to even call him great.
Terrible defense and dude needs to shoot a lot to score big, which will not happen with Westbrook and Durant in the team. Plus he simply cannot create and not as athletic as Harden, not to mention being injury prone. He is further limited with the new NBA rules such as kicking you leg out when or jumping towards the defender to get fouled, that is like half of his ft attempts right there.
Money speaks though and Harden turned down $52mil for $78mil the Rockets offered. But to me and hard to say really because I would never see that kind of money in my life but I'd take $52mil, with a chance at a tile with my best friends that I'll grow old with. Again, that is just me, Rockets offered him more $$$ and a chance to be that #1 guy and to some that is hard to say no to.
In the end, OKC still shot themselves in the foot, they are title contenders and has taken a step forward every year, this could be their title year but now with a completely different player (who may not even be happy to come off the bench and with a big chance to miss half the season), it'll definitely be a step backwards.