Yes, I have heard of it. It's made by a company called ECS (http://www.ecs.com.tw). It uses a chunk of hard disk space to act as a buffer. The bigger the chunk, the higher the "X" rating. If you use 650 MB of your hard drive, the size of a full CD-ROM, you may achieve 100x speeds. That's pretty much the same as copying the entire CD to your hard drive.
Anyway, it's a bit of a cheap trick. The drive isn't really that fast. It just supports a crazy driver that makes it act like it is. You don't get anything for free. You will give up part of your hard disk. I'd be wary. It still sounds a little too good to me.
This info is from http://www.geek.com