Do most prefer to run the RioCar to another HU, or use the RioCar standalone. Those of you who have possibly tried both ways, what are you happiest with? Ergonomic wise? Feature wise?

Either way will work. There are extensive discussions in the FAQ about how to hook it up either way.

For me, I look at it this way: Those who have CD players in addition to the Rio Car, usually say that the CD player gathers dust. So why bother even having one taking up space and looking ugly in the dash?

The Rio Car also has that 20-band parametric equalizer that I'm so fond of, so I have no trouble with it being my only head unit.

I bought the Rio Car specifically because I never wanted to carry another CD with me ever again, so having a CD player too would be a waste to me.