I don't own a 6D but when I got my 70D this past September, SterlingTek did not offer compatible batteries yet. I have used plenty of SterlingTek batteries with my 40D and never had a problem, so was a bit disappointed in the thought of shelling out big bucks for OEM batteries!
I ended up ordering a couple Wasabi Power LP-E6s and a charger and am happy to report they have been working perfectly!
One point worth mentioning is if you order aftermarket batteries like SterlingTek or Wasabi Power, be sure to get their charger also. Canon chargers seem to not care for non-Canon batteries and will show "full" on the charger but loose charge quickly if not used with their own charger.