They work great as an addition to fenders...as I have found fenders are not 'long' enough to cover water splashing from the wheel turning. They were easy to install, although you have to drill a few holes in your fender, the mounting provided is perfect and the leather is great at keeping flying crap away.
I installed an Old Man Mountain Sherpa rack on my Haibike Allmtn Full Seven Plus four (or more) years ago. It is a very strong rack, supported by the axle. (You probably will have to replace your axle with a longer one, as I did, which Old Man Mountain also sells.) I carry panniers loaded with professional photo equipment, including three cameras, long telephoto lenses, and tripods, as well as backpacking chairs, and burly bicycle locks and cables.