If all you want to do is play steam games then I'm sure steamOS is going to be the best experience possible. If you want to use it as a regular PC it probably works reasonably well but a user who doesn't want to use the terminal is more likely to run into a brick wall at some point (e.g. connecting to a printer or something). Something like Linux Mint is going to give an overall friendlier experience for someone new to Linux even if running steam games on it is slightly less friendly.

Ironically connecting a new Brother printer was the most painless thing I've ever done on Linux, because I didn't do anything at all. Linux saw it appear on the network and it just worked.