I should have picked up an extra one when they were still on sale...
Once you overcome the admittedly steep learning curve, it becomes the without a doubt, the most useful controller a PC gamer could own. It's got decent build quality (I've had it for 4 years now and it's outlasted 2 XB1 Elite controllers while being less than half the price) and is a huge boon to anyone who uses their system as an HTPC as it almost entirely eliminates the need for a cumbersome keyboard and mouse. Being able to navigate the OS, zip around Firefox as if I was using a mouse and power down afterwards all from my bed across the room literally changed my life lol. As a value proposition, the Steam Controller was a no brainer. At the same MSRP as a standard console controller, it offered great ergonomics (about on par with the XBONE controller), two extra buttons in the form of back paddles and an absolutely mind boggling amount of customization. The sheer number of community made configurations are truly absurd.

In regards to gameplay, I still primarily stick to mouse and keyboard with side scrollers and fighters being the main exceptions but where this thing truly shines the most in my opinion is in 3rd person action-adventure RPGs like The Witcher series. It's a lifesaver having certain sub-menus, skills, abilities etc mapped to certain buttons as opposed to having to remember keybinds for every little thing. I never was really a fan of big picture mode tbh but the fact that I can totally ignore it and not lose any functionality is great also.
I suppose if I had to nitpick at something, I do find the shoulder bumpers and rear paddles to be quite loud when pressed. Its not a dealbreaker or anything but it is the one department that I would say standard controllers definitely have it beat in.

I really do hope that Valve haven't totally abandoned the concept like they did with Steam Machines. It sucks that they launched around the same time because this controller was actually an amazing product that had the unfortunate distinction of being tied to Valve's biggest hardware failure so far.

Fingers crossed for a v2 or revision some time in the not too distant *insert valve-time joke here* future!