they work, they are practical. The main thing is taking the time and effort to determine just how much reinforcing to use, where to use it, and how much the wood can be reduced to compensate for the weight. CF is strongest when bedded in epoxy, so getting the right ratio, getting it cured properly, and in the right place is not trivial.
A good example of CF reinforcement is Randy's nose construction- two layers of 1/8 in. medium balsa with a layer of CF veil in between and glass or CF veil on the outside. Stiffer and stronger than anything you can do for the same weight using plywood and glass, but it took quite a bit of testing to get it right. Not too many people want to put the hours into a stunter and find out that the wing folds because the CF delaminated!!!