They're ok. I prefer the Verbatim DL media, but I bought a 25-pack of Memorex a few months ago because they were so much cheaper. They were made by Ritek. I have burned all 25 without any trouble. One of my players has trouble recognizing Ritek DL media, but the disks all play fine on my other players. I mostly use Verbatim for DL media because I never have any trouble with them.
If you can afford Verbatim, I'd recommend those instead, but if you can't, Memorex will probably work fine. I guess it depends a lot on you burner and player. Good luck.