I remember discovering these on a deal a day site for parents a while ago and thought they were kind of neat. I particularly liked the Halloween, Classical and Lullaby ones, and had every intention of buying them for my daughter at the time...but never did because we already had so many other CDs.
Honestly, a lot of the music seems like it would get annoying fast (as a parent). It's a GREAT deal, especially if you break it up and give the CD's away as gifts, but if you want something your kids will stay interested in, I'd say buy a couple of Eric Litwin CD's (the guy who wrote Pete The Cat: I Love My White Shoes). His songs are all done in different styles (anywhere from Zydeco, Blues and Jazz, to Rock, Folk and Disco). My kids stay entertained a lot longer with it than any other kid music we own (they especially love the song "Stop, Drop and Roll" and his version of "The Three Little Pigs"), and my husband and I can actually stand to listen to most of it, too.
$15 is great, but only if the CD's aren't sitting on a shelf collecting dust after one week.