I had one of these installed in my car by a local car stereo shop last weekend. They did a great job, but by the time I had them add a backup camera and the necessary controller to make the steering wheel buttons work, it got pretty pricey!
That's okay, as it was my Christmas present to myself.

And so far I'm really enjoying it. I picked the Alpine over the Pioneer 4000-NEX because that one was junked up with a zillion awfully designed features and interface elements I would never use, and had a worse touch screen and no dedicated Siri button. (It did have HD radio, though, which would have been nice.) The Alpine is better for me because it is a radio plus CarPlay plus a nice screen, and that's it. Much more elegant and Apple-like. Plus, the dedicated Siri button is great to have, as it's very easy to find and press while driving.
Overall, CarPlay works great and is easy and fun to use. Siri generally figures out what I want. I've been using maps, sending texts, making calls, and listening to music on my iPhone 5s. Oh, and radio, too.

I would love to see CarPlay continue to expand and get more features and third party apps, so I hope Apple will remain committed to it. We'll see how my gamble pays off.
A few more notes for anyone considering one of these:
- Lack of Bluetooth is a non-issue. It's super easy to plug in your phone when you get in the car, and that gives you a great connection and charges the phone as well.
- You can listen to radio and use maps at the same time. I wasn't sure of this so checked before I bought the unit. It's not HD radio, though.
- The microphone is an external unit that needs to be mounted somewhere in your car, like by the rear view mirror.
- Adding a backup camera and integrating with your steering wheel controls will dramatically increase the cost, but it makes the unit feel much more integrated with your car.
- Lack of scrolling and panning is a little odd, but does seem like a conscious decision. It doesn't really harm usability. It's easier to ask Siri to play a certain song or artist rather than scroll through a list.