Junkman,
The car lift is one of those extravagances that I'm sure glad I bought. I've got a restored musclecar ('69 camaro) that needs constant upkeep; for example, last winter I pulled the motor and changed the rear main seal and oil pan gasket. A guy could do this with jack stands and a lot of time on his back, but a lift sure does make it faster/easier/more pleasant.
Another example, I can lift my JD3320 and pull the mower blades without pulling off the mower deck, or jacking up the tractor. If I were a little more precise with a 4" grinder, I could probably sharpen the blades without removing them.
Another thing that comes in handy is that I use the lift to store my car. I put it on the lift, jack it up and leave it there for the winter, and can still use the space underneath for another car, or other misc. stuff (like lawn mowers, snow plow, etc.). My tractor will NOT fit under the lift with the ROPS extended.
To lift a car to a comfortable working height, you probably need 12' ceilings. If you're short, or you don't mind stooping a lot, you might be able to get away with 10'.
If you decide you want to build to suit a lift, consider a 2 poster. They're better suited to working on cars because you can get to wheels, suspension parts, etc. 2 post lifts do require significantly more sturdy floors than 4 posters, and a bit of installation. On the other hand, 4 posters can be equipped with wheels (which come in handy).
-nosualc