The ceiling tiles give off a dust and fibers that I seem to react to, at least I had a hard time putting in the drop ceiling in my basement. Apparently most people aren't bothered by them though.

I use homosote over plywood. I have two benches, one wedged in next to the water heater and bolted to the wall, and another on wheels that I roll around the basement to get it out of the way when necessary.

The one by the water heater is screwed to the wall, and at least the back part is very straight, and as long as the wall doesn't sink in to the ground too far, it should stay that way. The rest of the bench is supported on some metal angle to keep it straight.

The workbench on wheels is built out of 2x3 and 2x4s with the plywood decking. It's actually rather flat, though not 100% perfect, it's close enough.