What I find funny is how all of the negative '5D3 isn't worth it' replies are coming from people who do not actually own the camera.
I think it is valid to make up one's mind based on tech specs, iq comparisons and reading many reviews, and I don't think having something in your hand magically adds something to this apart from the fact that all current dslr bodies are "good and fun to shoot with".
Properly done reviews are useful, but nothing takes the place of actually trying it out for yourself. It's kind of like reading a review on a new Ferrari. Sure, I can read a review and make a call whether it's worth it, but ultimately if I need to know what the car is truly like I need to get inside one and drive it.
I read quite a bit of info and looked at a lot of sample images of the 5D3 before mine arrived, but there's a big difference between seeing someone else's ISO 12800 shots and examining your own. Only when you integrate the camera into your professional workflow can you truly see how well the camera fits you.
Keep in mind that my definition of "worth it" is entirely a professional one. The way I see it, if the income I make from shots not possible (or not sellable) with my 5D2 but possible and sellable from my 5D3 exceeds $3500, then the camera is worth it. So far, the combination of high ISO performance, much better AF, and little things like the viewfinder leveler mean that for my uses this is very likely the case.
For the hobbyist this is a more difficult decision and depends on where you put your values. Even a hobbyist, though, will not truly understand the value of the camera without trying it out.