Once installed, the fenders work well, much better than most designed for mountain bikes. I fitted them to my Marin Hawk Hill with 26" rims. The rear fender has only about 1 cm of clearance at the closest point. This should be sufficient on roads, but probably not for muddy conditons...but then, if you're a serious mountain biker you probably won't install fenders in any case. I've used them once in wet conditions and they worked very well. The front fender is much better than the type that strap to the frame, as it keeps water spray off your legs even when turning corners.
The front fender was easy enough to install but it's very fiddly to use the locking mechanism unless you have very small hands. The rear fender, though, can be installed in a couple of minutes. Overall, these fenders are well made and work much better than other ones I've used for mountain bikes.