Allright, you've got me interested, what were their gaming habits before? Because the way I'm looking at it, outside of some niche cases for streaming, the steamboxes are aimed at people who don't play PC games.
But I'm biased because I, and all my friends, are PC gamers.
They owned an Xbox 360 and played games like Left4dead 2, Halo 3, and Street Fighter IV primarily. Once they got their steambox and saw that Left4Dead2 and Street Fighter IV were there, and the sort of sales they had, they switched. They were already looking for something to replace their Xbox 360, and the PS4 and Xbox One hadn't launched yet EDIT: Scratch that, they had launched, a few months prior. Getting a steam machine for them was awesome. My friends have since played through Half Life 2 and the episodes and portal and sleeping dogs. They play all this with either a steam controller or a wired xbox 360 controller.
I have another friend, life long playstation gamer. Earlier this year, he asked me to help him build a PC after hearing about my trip to dev days and dinking around with my PC and PS4. I put together a $750 build for him and he's been blown away since. In fact, checking steam right now, he's logged in using XBMC. He's taken to games like Metal Gear Rising and XCom and CivV and he's interested in Elite Dangerous after trying my setup. Had Steam Machines been available for consumers, I would have steered him to get an alienware build instead of building his from scratch. He uses an Xbox 360 controller with a dongle.
I think this idea that PC and Console gaming tastes are wildly different is outdated. There is plenty on steam to satiate the console gamer.
EDIT: Neither of my two groups of friends here are what I'd call hardcore gamers. My second friend, the one into MGR and XCom, is more so of a hardcore gamer than my first group of friends, the one with an actual steam machine, but they don't visit forums or anything like that. My friend, the one who plays XCom, visited me a week ago and asked me "what is family sharing" and I have no idea how he heard about it. Regardless, we've shared libraries since and he thinks that's awesome.