I have the Tamron 24-70/2.8 and a 6D, and I love the combo, save for one thing - My lens is currently on it's way back to Tamron for a firmware upgrade, as there seems to be a bug between the 6D and the lens that allows the lens to draw power even if the 6D is switched completely off.
I was getting battery drain at the rate of 4% per hour with the Tamron attached. If I put my Canon 50mm/1.4 on, it doesn't drain at all. (before anyone says it, the wifi/gps are all off)
When the lens isn't draining the battery, it's a pretty wicked combo - focus is smooth, fast, and fantastic in low-light. The IQ is great across the range, better than the Canon 24-105L if you ask me.