They're great. Inexpensive, reusable, easy to install, and very effective.
Downsides are they're heavy, need a little scissor work and a bit of duct tape to install, and increase rolling resistance. The only issue I've heard of was with running tires at a very low pressure, they may chafe the tube.

Be sure to trim the edge to a nice curve, and stick a layer or two of duct-tape over the end.