Upload & Sell: On
It's best to think of the 5D3 as a 5D2 with great AF and much better video. Are these major things for you?
There are other tweaks, but they're not huge. It's still 5D2-era sensor technology, as can be seen by looking at the 6D and Sony/Nikon/Fuji. You can't get a high precision focusing screen. I think we're on the verge, after a long period of incrementalism from Canon in all but the 1DX-- another big jump. But it's going to take 12-18 months (just a guess).
I think the main questions to the OP are: What do you have? What doesn't it do for you that you want/need it to? What do you hope to get from getting a new camera now (or in the future)?
If I were buying new right now, at this point, I'd strongly consider a 5D2, 6D, Sony RX1, or something mirrorless from Fuji/Sony. Might still end up with a 5D3. But not before thinking about i) my wants/needs), and ii) the potential to satisfy those wants/needs elsewhere while maintaining my ability to get back into the 5DmkX if there's a big sensor technology jump in the next round.
I'm actually doing that. I've got a 5D2 that is my professional workhorse. In the meantime, I've got an X-E1 and lenses to carry daily and explore other options with. Now I'm eyeing either FF-mirrorless (RX1, or mythical FF-NEX/Fuji), or return to 4th gen 5D body depending on what it brings.
It's a great time to be a photographer!