I got one for CDs and 2 for DVDs, and they hold a lot of both, but with DVDs, I get 5 rows and then the bottom is too short for 1 more row unless you lay them down on the front edge, but I like to see the titles.  As for CDs, I get 8 complete rows with almost no wasted space.  As my collection grows, I would certainly buy more of them.  
Ideally, you want a CD to fit just perfectly on a shelf with enough room to fit your finger in at the top to get them out, and them make it so your shelves come out perfectly.  With DVDs its a little trickier, since some DVDs have bigger packaging (usually collections).  All in all they are great shelves and very versatile.