Seen it done many times but it just wouldn't work for me b/c I would spend too much time hosing down the floor and uncloging drains.

I would caution you to have a slight slope to the floor to allow for drainage and don't trowel it to a glass finish or you might slip and fall.

The only thing I can think of as a disadvantage would be you cannot plant directly to ground in concrete. You can discount the expense since the pad is pre existing. Concrete would be easier to keep clean and keep weeds from sprouting in the corners and under you benches. I am sure contrarian views will be along shortly.... :)

Thanks folks for your input! We had seen several GH pics on wooden decks, but wasn't sure about concrete......... I would like to have beds, but it's not a deal breaker...... At our age we're always looking for ways to work smarter not harder! ; )