I got mine this last summer at Walmart. It was like $13 or so. I had the one listed on that redoctane site, too. Couple things I want to say about it.
First, I really don't like it. XBox controllers sucks for anything that's not a FPS, so I figured I could get sports games for my XBox while still using the PS2 controller. Most games out there do not allow you to put set any action to any button...they're usually like 3-4 preset layouts you can use, which sucks in this case. If I remember correctly, the right trigger on the XBox corresponds to R2 on the controller...I'd prefer it was R1, as R1 is better for shooting and for juking in Madden! (And you might be able to swap those in Halo and Madden, never looked into it, though.)
Also, I bought a GameCube in early December, and the converter just flat-out did not work for the GameCube. Nothing happens when I have it plugged in. No reponse or anything. That's what I really wanted it for...because I despise the GameCube's setup.
I can't really say I recommend it, but I can't say it sucks. I guess if you're desperate to use a different controller, go ahead, but don't expect something great, because I sure as hell didn't get anything I'd call great..