I have to agree with all the people who have written above claiming it is worth it. I upgraded from 7D to 5D.3 in May, and I had my doubts before the upgrade. A lot of reviews compared the 5D.3 to the 5D.2, hailing great improvements, but the few comparisons to the 7D was not as convincing. For instance, a web-site wrote about the advantages of the Q menu, compared to the old system on the 5D.2. But the Q menu was inherited from the 7D. Also, the 5D.3 is more than twice the price of the 7D.
However, as soon as I had the 5D.3 in hand and started looking at the menus and took some pictures, I knew I had made the right choice. There are tons of extra features. In low light situations, I have 3 extra ISO stops to work with (from 1600 on the 7D to 12800 on the 5D.3 for decent pictures), but also with good light, the camera seems to render contrasts and details better. The best part of all: The autofocus system. It is both faster and much more accurate that on the 7D.
The only disadvantage I see, is that the upgrade is expensive. One thing is what you pay for the body, but the 5D.3 deserves good glass and most of my old glass felt unfamiliar with the new crop-factor. My 10-22 EF-S had to be replaced, and I still wonder what I should do to get closer than 300mm. On the 7D, my 70-300 always got me close enough. Now, I want something more, but anything beyond 300mm is horrendously expensive, or just bad.