The ONLY downside is what Westman Farmer mentioned.  But properly installed it shoud not be a big problem.  #1 Most important is insulate under the slab properly.  Bubble wrap or foil is a waste of time, effort, money and energy.  At least 2" insulation all the way around.  Zone the area.  If you are making separate rooms each should be a separate circuit with its own thermostat.  You can also run pex between the floor joists to heat the upstairs.

The first time you jump out of bed in your bare feet and hit a warm floor you will wonder why you didn't do it years ago.