I have a Steam Controller, and I don't know if it's just my experience, but I found it takes a significant amount of (user-friendly) tinkering to get it to work the way I want it to (which is annoying), but when I get things set up the way I want them, I like it a lot.
In some ways, I feel like it's better than a mouse and keyboard, even at a desk, because to me at least, it feels like I can react faster to what goes on in games when using a controller, except when it comes to aiming, and regular controllers suck at aiming. This controller fixes that problem to a very large extent, but still not quite as well as a mouse does.
However, at least for me, it's a no-brainer to use a Steam Controller as opposed to a mouse and keyboard when "couch-gaming" (even if you have a piece of wood on your lap). (For one, when I have something on me that restricts my freedom of movement, it's like all I want to do is move. :P)
Ultimately, I guess a good "rule-of-thumb test" to see if you would like a Steam Controller is asking yourself, if a regular controller aimed better, how much more comfortable would you be with one as opposed to a mouse and keyboard setup on a piece of wood.
For what it's worth, though, I have an Xbox 360 controller, a Steam Controller as well as a mouse and keyboard (which I use at a desk), and I just use whatever is the best for any given game, but if I only had to choose one device for a gaming-only system, then I would choose the Steam Controller.
Hopefully, I've been at least a little helpful to you. :)
Edit: It does feel a little cheap to me, but it doesn't seem so cheap when I check it out. I think the cheap feeling comes from the fact that it weighs less, and we associate heavy as heavy-duty/strong/tough/etc. For one, I think the controller's lightness helps make the aiming easier.
@Phazevariance: Then, Microsoft could just refuse Steam into the "Universal Windows Platform" store, and find a reason to "explain" why they would be doing so.
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