I have a minor case of buyers regret with my gripped 5D3. It is a very competent camera and delivers brilliant files, but as a long term user of 1-Series EOS bodies pre-dating digital, the handling, ergonomics and sheer class of the 1-Series is sorely missed.
Business is pretty strong at the moment so a 1DX is an inevitability here. I wish I'd held out and waited for the 1DX.
One unexpectedly high use function on the 5D3 is the silent shutter. When shooting boards of directors for corporate clients, I encourage them to give me 30-40 minutes of real time during a board meeting, with the agreement of no-flash use. The silent shutter has enabled some sessions to stretch out to an hour shooting with 70-200 f/2.8 & 300 f/2.8. Without the clack of a regular shutter, I can almost disappear in this environment, despite being the elephant in the room. Heck, I just leave the 5D3 on silent.
Other than the silent mode, I'd have the 1DX any day. And if the current level of business is maintained, make that two 1DX bodies. After extended test-drives in revealing conditions, I'm totally gob-smacked by the 1DX whereas the 5D3 leaves me with a feeling of, "Yes it's pretty good, but...."