Never lived in a house with floor vents. About to move into a place (Austin) that has them and wanted to know what, if anything, to expect different from traditional ceiling vents.
Aren't they just returns, or are all the vent faces in the floor?
I had an old house that had "aftermarket" air installed and only the returns were in the floor (vents were in the ceiling). The only problem I had there was that it meant the furnace-dohickie was in the garage taking up valuable space. Newer construction has all of that in the attic.
Here in Colorado, nearly all of the vents are in the floor. It's great in the winter, since hot air rises, but sucks in the summer.
May be a good idea to get some vent filters to prevent dirt falling in the HVAC ducts. Not a huge deal, but it helps keep them clean. Of course, they block the airflow a bit, so it's a tradeoff.