Which version of Mandrake? I have to say 9.1 is the first linux distribution I'd reccommend to non-geek types. Especially as a replacement to windows. It's the first linux distribution that allows me to plug in my camera and access it without having to open up a shell and mount it.
You can learn any OS as much as you want. Debian will force you to try figuring out how to get all the kernel modules to work and may be hard to set up. Mandrake will do everything for you. But the fact that mandrake looks nice (and it does) doesn't mean you can't explore what's happening behind the GUI. You can just do it when you have the time. Nothing is obscured in any way. You don't have to learn anything with debian either if everything just works. It's not the OS that matters, its what you do with it.
I'm not saying Debian is bad - it's running on my server and I'm very happy with it. I'm just saying that there's no point in installing something that may take you hours to set up (if it doesn't recognize your software), and then hours to download the latest files to get everything to work. A lot of the hardware on my desktop works only with the latest drivers, which mandrake includes. Debian stable does not. That's not a problem for my server, because it doesn't have a fancy video card or sound card. But on my desktop I don't want to have to spend hours trying to figure out how to get my sound card to work well. Mandrake picks it up at install and I'm done.
If you really want to learn how Linux works, install LFS. I've done it. It's not fun, but you can learn a lot, plus get a very lean system if you do it right.