I'll try to answer some of the questions above based on my experience. First, my garage is 20'x20' and I did it with aluminum soffit material (post #12 above) so it's neither steel or drywall.
Cost - My total cost was a about $475 for the panels and screws. I didn't cost out drywall but my guess is that it might have been as little as half of that even with the cost taping, mud and paint.
Labour - I did mine myself without having to gut the garage of tools, equipment, etc. to do it. It went up very quickly and there was no drywall lift, taping, sanding, painting, etc. to deal with.
Noise - I can't compare it to drywall but I haven't found it to be a problem at all. The loudest thing I do is probably grinding metal and nothing is going to change the noise that makes.
Lighting - It's great but I can't say that it's any better than drywall painted white would be. I initially bought 4' T8 flourescents with deflectors. I found that the lights without them work better as the ceiling itself is a good deflector.
Durability - My ceiling is 10' high and I do have to be careful swinging lengths of metal around. I have flourescent lights hung from the ceiling so I have to be careful anyway. If I do damage a panel I can replace it but it would more difficult than patching drywall.
Weight - There is no comparison here. My entire ceiling probably weighs less than 2 sheets of drywall.
Building Code - I don't have a habitable room above my garage (attic only) so I don't believe this is an issue.
Vapour Barrier - I know there are differing views on this but in this area it's common to install a barrier on the walls and ceiling. The attic is well ventilated and I don't expect any moisture, mould, etc. problems.
Paint - The panels are matt white and are good to go as is. They are designed for outdoor use so I can't see that they will ever require touching up due to dulling, peeling, etc.
"J" Molding - Intead of using "J" molding I installed a band of 1"x6" MDF to finish of the corner seams. I use it to mount my air and electrical conduits to.
Appearance - I feel that it has a more industrial look which I like.
Overall I am very happy with the way it went up and turned out and am glad I went with aluminum.