CarPlay is cool, and if I was buying a car that had it, I would be stoked to use it—but I'm not going to upgrade the stereo that's in my 2006 Subaru to this aftermarket Pioneer system.
For starters, CarPlay can't do anything my iPhone doesn't do on its own, and obviously the iPhone has a lot more apps. I really missed using Waze for navigation—I could still set up a Waze route on my iPhone's screen and the directions would play over the car's speakers, but since it's not a CarPlay compatible app it can't show the map on the stereo's screen.
Add in the high aftermarket cost, and the hit-and-miss results from using Siri for everything, and CarPlay doesn't seem worthwhile just yet. But navigation and hands-free communication work well, and it's nice to have an Apple-designed interface on a car stereo. Since it's so easy to update the software and apps on your phone, CarPlay should keep improving. It's already better than other infotainment systems I've tried—and a lot safer than trying to use my phone while I drive.
I haven't yet played around with CarPlay—dealer-installed or after-market—but Susie's experiences seem pretty much on the money. Limiting CarPlay systems to Apple's default apps makes it a tough use case for folks who prefer Waze or Google Maps to iOS's mapping program, and I have yet to meet a car touchscreen I've been impressed by.
So while I hope that the car manufacturers put some care into CarPlay integration, I'm skeptical. The CarPlay software is really nifty, but there's a reason Apple likes to build hardware and software together—it works a lot better that way.
In fact, when I got my new car, I intentionally went with a Bluetooth-capable model with no touchscreen (and lots of touch-friendly knobs and buttons). It may not have CarPlay, but it has my fake version, "CARDISPlay": a charger, mount, and "Hey Siri" on my iPhone 6.
It works pretty well, even with high road noise, and I find myself hardly ever needing to tap or swipe on the device. And when I do swipe—to pan around a map, for example—it responds quickly, as iOS devices are wont to do.
If you prefer having a big touchscreen in your car, the Pioneer (or a built-in system) may well be the way to go. On the other hand, the iPhone 6 and 6 Plus are practically tablet-screen-sized... Adding one of those devices together with Hey Siri and a good mount might be a decent alternative.