I've owned a 12" Retina MacBook for about nine months and feel rather ambivalent about it. It's surprisingly powerful and effective for work, but Apple seems to have given up on the USB-C ecosystem at launch. The single power/USB port severely hobbles it, Apple's only made a couple of comically inappropriate adapters, and the third-party options are outright trash that doesn't even work. Enter OWC's USB-C Dock—loads of useful ports!—which Glenn Fleishman says is the first thing worth buying. But you'll have to pay $160 to get it.
If you haven't purchased a MacBook yet and can wait, it's worth looking at Apple's next laptop releases. Thunderbolt 3, which will use USB-C as its connector type, is due out later this year and is already announced for a special Dell developer edition laptop model. Apple's roadmap is unclear, but it's likely we'll see a revised MacBook at the March 21 event or in April, as the first model appeared just over a year ago; it's just possible it'll have Thunderbolt 3 included. Other MacBook models will likely see updates this year, too, and Thunderbolt 3 is a natural for the Pro.