I think the extra cost of the 535d is worth it if you like the feel of a powerful car. I'm trading 'across' from a M5 btw. I had the 530d as a demo for a weekend and it was a great car, enough power for most people most of the time.
However the 535d takes it up a significant level, it goes from a nippy to a genuinely fast car. It will also put a big grin on your face and make your heart race a little faster everytime you floor the the gas pedal. If this sort of thing is important to you then the 535d is a must, if you find this sort of thing immature and unnecessary then the 530d is a fantastic car as well.