I think for someone starting out and making smaller projects, they are a great idea. Without a whole lot of money, you can get a wide variety of fabric. They are great for smaller projects, or for "scrappy quilts", or for quilts that don't repeat block selections. They are a great way to increase the color options you have without having to mortage your house.
I find the more I quilt, the more I find some of these fabrics that I want more of to do a larger project or more projects with. And, I find some of the fabric that I wouldn't ever want to use again (due to color fade or pulling one direction, whatever) and I'm glad I didn't waste money on a huge batch of it.
I think it really depends on how you quilt.