Hi,
Alamo is similar to National in that you can pick out a car from the car class you had reserved. Many times you'll see National cars in the Alamo spots - they seem to share the same fleet.
You don't go to the garage unless you're a member of their QuikSilver program. Otherwise, if you are a US citizen and have a reservation, you can either use their on-line check-in process or their kiosks that are available at the counter (of course, you could always stand in line.)
Drop-off is simple, and Alamo/National share the drop-off area.
I tend to see 'nightmare' stories on these boards about Alamo, but I had a great experience with them at the cheapest rate around.