They're terrible, don't buy them.
If you own the new 12" Macbook, you have one USB-C port. It's also the power port, which means if you have anything plugged into that port, you're losing battery charge. To both stay plugged-in and operate a USB peripheral—a backup drive, say—you have two choices: an $80 AV adapter from Apple that does things you don't need, or an enticingly cheap $20 third-party dongle that supposedly does.
These devices pass power though to the laptop while adding extra USB ports.
They even come in a passably-accurate shades of the new gold and space gray colors. Any good?
No! They suck. I've tried a couple of brands now and they're the same. The problem is twofold:
• They only pass through old-timey USB power, which isn't enough to charge even a small laptop before the second coming of Christ. Under heavy use I would get the little message saying "battery not charging," which I take to mean it wasn't even power enough to maintain the charge it had.
• If you plug in a power-hungry device such as a bus-powered USB hard drive, they go haywire. The USB disconnect/connect chimes go on and off and the drives makes sinister clicking noises.
Various improved products are supposedly coming out soon that remedy the shortcomings of these generic USB-C crapgadgets. But in the meantime, it's not worth buying them if you expect to use them for charging. I'm not even going to Amazon-link it ironically, because I'm making an executive decision for you: do not buy.
Come now, Apple: make us a better USB-C mousetrap.
Have you found a brand that actually works? Tell everyone about it in the comments!