Tonnemaker: I don't have a controller for my PC right now and I've considered getting the Steam controller because it looks like it would be comfortable to use. Is it? At some point, I'll definitely get a controller and it's a strong possibility.
Oh, yes especially if you like Haptic Feedback pads, they reduce strain of your thumbs. There's also loads of interesting settings you can change. For example, in Witcher 3 you need a lot of buttons, more than there's available in one "setup" for the SC but you can add another schematics that you then stick to one button so when you press it you instantly get a new "setup". I have one for Exploration and one for Combat. You can change sensitivity on some of the buttons too.
Then there's the HF you can add out of boundaries hotkeys so if you touch the edges of them, you for example add a Run hotkey making it more seamless rather than pressing or holding yet another button somewhere.
It's still inferior to mouse if you want to play fast strategy or shooter games.
leon30: How do you rate it compared to the ps and xbox controllers?
It's better than both IMO but I have limited experience with a PS2 controller (I bought for PS1 and PS2 emulators) and even less with xbox, I don't really like that one.
It boils down to the Haptic Feedback. You can actually use them as physical buttons but I never do, the experience of having your fingers flow between the buttons is great.