I upgraded from a Rebel t1i to 5D 3, here is my personal view.
Is it worth 3.5 large? No.
I am not going to bother comparing it to my rebel because that is just a waste of time, 5d3 is targeted at different people. But I might as well compare it to D800 since that camera is meant to go head to head against the 5d3.
While D800 and 5D3 go neck to neck in DR and features as the most reviewers are finding out, I don't see why one costs $500 more than the other, while not being better.
So simple as that, it's not worth 3.5 large, but if it had the same price as D800, yes it would be worth it.
With that said, I definitely don't regret buying a 5D3 over D800, because for me, the primary deal breaker was the glass, not the body. Having shot with Canon's 70-200 II and Nikon's 70-20 VR II, I definitely prefer Canon's version for IQ. And although my preordered 24-70 II isn't going to be available until July, I have a feeling it will have better IQ than nikon's current 24-70.