Does what it says on the tin, and then some
So, this does exactly what I wanted it to do, which is mix my TV, HTPC, phone, and anything else straight to my speakers, with no power drain, heat production, or cross-channel feedback/noise. However, it should probably be noted somewhere on the product page that you can combine the splitter/mixer functionalities together, but not 100%... I'll try to explain in words, the best thing would be to include the diagrams that are on the back of the package, which give a much clearer visual indication of the secondary mode.
So, the sockets on the back are in two rows of 5 ports each, with the last port in each row being connected to all 4 of the other ports. Those last two ports always function as the mixer/splitter they are labeled as. The other four ports, however, have switches built inside the sockets, that check whether a plug is inserted. The plug doesn't actually have to be wired up to anything, just physically in the socket. If a plug is in both the input and output socket for a given column (not row) the sound for that input will go vertically upwards, through the volume knob on the front, out to the port directly above the input, and NOT to the mixed output at the end of the horizontal row. This lets you, for example, mix three ports and use the remaining volume knob as a pass-through line volume adjuster, without affecting the other three. It also means if you run your entire setup in 1/8" stereo cables (like I do), and buy 10 1/8" to 1/4" stereo adapters, and plug all 10 adapters into the box so you can patch in 1/8" cables as needed, the mixer will NOT work, due to each input being routed upwards instead of horizontally to the mixer output. Once I figured that out, I unplugged all the adapters I wasn't actively using and it worked like a dream.
There's also a tiny bit of output volume drop when you patch in each new input for the mixer (I can't speak to the splitter, haven't tested that), but it's way lower of a drop than the old mixer I had, and isn't really noticeable unless you're specifically listening for it. However, if you want to keep your line levels absolutely perfect while swapping inputs, you'll have to adjust the knobs a bit to compensate.
The overall build quality is incredibly solid, it's the kind of thing you could toss in a bag with a bunch of heavy microphones and worry more about the mics than the mixer. Knobs are smooth and unnotched, and because it's passive there's no worry of leaving your gain up too high when plugging in sensitive equipment.