I've plumbed two tanks on my main floor into a basement fish room, both times using hard tubing. For my next tank I'm seriously considering using spa-flex, and have read enough to know that there aren't any big danger signs that I'm missing.
I've always thought that hard pvc pipe was pretty easy to work with, but being limited to 45 or 90 degree turns has added lots of "hard" turns to my plumbing, thereby adding resistance to the waterflow. I think if I used spa-flex the curves would be more gradual and result in less head-loss/resistance.
The only advantage that I know of for hard pvc is that it'll contain higher pressures, but that isn't an issue for most aquarium usage.
Hopefully a few more people will chime in.