i love them because i have a faddie and i can only get a really good fit with them. they do tend to wick if you put pants on over them and your little one's sitting in a carseat, highchair, whatever.
i especially like that they are stretchy and don't leave marks on big thighs. also, they wash well, and look new longer than the average cover, as long as you don't put them in w/prefolds.
imo, other than the occasional wicking, they're worth the 15-18 bucks.