Not a scam really, but they are not very up front about the charges. Sure, you get 6 free CDs at first, but you pay shipping and handling on each CD (and tax), which is about $2.49 a CD, of course, the package they ship it in looks like it would cost $4 at the most, so they do profit from you from these "free CD's". The one CD you do have to buy is marked up to $7-10 above store costs, plus shipping and tax, so what should cost $11.99 or so ends up costing $23-$25. You also have to buy the CD within a year.
Something to watch out for is the shipment of a CD you didn't want. The last CD service I was with (which is now defunct) sent me a CD without me asking, and I ended up paying for it (I gave the CD to my mom). Afterwards, I got certficates for the 4 remaining CDs, which again, you have to pay shipping and tax for.
I mean overall, you do save money, even though you get so many offers you don't want, and may end up with a forced purchase, but you do pay for only one CD (and an assload of shipping charges that sum up to $25 or so).
One problem you may have is with reselling the CD's. Stores won't take them, as they those subscription companies have the barcodes changed, and may even say something like "Distributed by (whichever service you're with, like BMG)". So once you buy them, you're kind of stuck with them, unless you Ebay the CDs.