Mackie HM-4

I use this a lot more than I thought I would

Works great in any scenario where you need more volume in your headphones or a speaker with a headphone input.
To my ears it slightly brightens the sound, but I use it in specific scenarios so take this with a grain of salt.

I got it to raise the volume of backing tracks when I'm drumming on my electric drumset, did the job.

I found myself using it to listen to music on my tablet with the studio headphones, WOW
It's now my favorite way of listening to music, when I want to hear the details.

Also use it to play podcasts, videos and music through my bass amp, it's just convenient.

Strong metal casing, knobs feel kinda cheap but work great.
Looks cool as well, I have to say

Negative aspects:
It's small and a bit heavy, with large jacks and cables plugged in it has a tendency to not be stable and fall.
That's the one note I have to give, just be careful so it doesn't fall and you are good.

Great product, my favorite studio tool

Pros:
Insane value for what it is. I upgraded my headphones to Behringer DT990 PRO 250 and I needed to power them. This gets the job done phenomenally. The knobs work smooth and there is no friction "electronic" noise when manipulating them. Plus my workspace looks a bit more complex now with another device on my desk.

"Cons" (funny):
If you keep it in the same room you're sleeping in, the green led light might be pretty bright at night. (LEDs are plaguing my setup at the moment...). This little LED seems bright enough to light up the ceiling area above it at night. I guess you can cover it with any sticker.