Rec'd my battery grip today (the "Amazon Choice" Vello BG-C8; not as cheap as the less expensive ones, but fantastic reviews and not nearly as expensive as the Canon brand.) If there's one thing the 6D excels at above and beyond the SL1 its battery life - one already lasts a half-day of shooting. However, I noticed during our Pin-Up shoot, back-button focus and shutter release while vertical was less exceptional, thus the grip, which mimics the AF-ON/exposure-lock/focus point selection as well as the aperture wheel and shutter release. The battery grip also comes with an emergency sled you can fill with 6xAA batteries.
I have potentially four shoots before the end of the year, two already scheduled. Up next, Medium Format and/or Hasselblad. Stay tuned!