They work, but if you have your stock airbox lid and some pantyhose, you can make your own!!
I ride in some muddy stuff, not wicked slop all the time, but some mud none the less, and have never had an issue. One trail that goes through a real swampy area for a long ways always floods real bad this time of year, and I've driven it with no airbox cover and water at about the shift lever and not had an issue with water. Obviously not flying through the water....chugging along in 2nd and sometimes first.