Let's get this out of the way; if you think it's crazy to add a pound of rotating weight to your bike to mitigate flat tires and rim damage, Cushcore isn't for you. If you've had race weekends derailed by punctures, had to replace multiple rims per season due to impact damage, or have resigned yourself to running 30psi+ to fend off flats, this setup is worth a serious look.
The Good: First and foremost, Cushcore actually lives up to the marketing promises. It acts as a bumper between the rim and tire under full tire compression, making it extremely difficult to pinch flat the tire's casing. Because the insert is much wider than the tire, it applies pressure to the sidewall of the tire, reducing the chances of burping air, and supporting the sidewalls in corners at lower pressures. It noticeably reduces the "pinging" sensation that you'll experience as the tire deflects off rocks at speed. The sensation of riding Cushcore is similar to the damping provided by dual-ply DH tires, although it's even more pronounced. I was originally skeptical, but the improved tire feel has become one of my favorite features of the Cushcore system. In practice, it has effectively eliminated pinch flats, stopped rim damage, and has plainly allowed me to ride my trail bike faster, especially in rocky terrain.
The Bad: It's an absolute hassle to install. Following the instructions and using soapy water helps, but installing and changing tires is a bear. It's heavy, at roughly 200g/wheel. It's expensive for what amounts to a set of glorified (albeit carefully engineered) pool noodles. The included valve stems are not long enough to fit many modern carbon fiber rims. And one insert lasts through about two tires before the foam has broken down to a point where it looses its supportiveness and must be replaced.
The Noteworthy: Although plenty of folks have claimed that they've been able to run lighter weight tires and much less pressure with Cushcore, that's not been my experience. Tire pressures have been reduced slightly, with attendant improvements in grip and rider comfort (26f/28r, from 28f/30r with 2.3-2.4 tires @ 175lbs). After experimenting with EXO casing tires, I'm back to running Double Down casings to avoid cut sidewalls. YMMV.
Overall, I'm very impressed. It's not perfect, and it's obviously not for everyone, but for those of us who regularly exceed the air retention capabilities of reinforced tubeless tires, Cushcore is an effective solution to one of mountain biking's most persistent problems.