Good points, all. As far as room with the garage door open - I built my garage with ten foot ceiling and eight foot high doors (standard is seven). That way, when a car is on the lift, the door will actually be over the trunk area, depending on the car. Obviously an SUV would pose a different situation - but things look good at this point.
Regarding the concrete: 12" thick with rebar? My gosh, that would be a little extreme. Specifications call for 4" thick, reinforced, 3,000 psi concrete with hoist posts at least 4" from any seam or joint. That means MOST standard concrete floors will meet that spec - and since I built this garage, I made sure it was set up for a two-poster.
My bigger concern is puttling the lift close to a side wall. I will have about 18" space between the post and the wall and I'm a little concerned about having enough room to work on brakes and wheels. One of my waiting projects is an axle replacement and I'll probably have to back the car into the bay so I have ample room to slide the axle out of the housing.