It's really what you make of it. It will most definately change your perspective on WDW, but that doesn't neccessarily mean it will ruin the magic- only change it.
I'm a character performer myself, and so far, I love it. There really is nothing like it. You get to actually be the one to create the magic that you love to receive so much. When you see that little child run into your arms with an ear to ear grin and give you a huge hug, you just melt!
You get used to seeing characters backstage in a way you don't usually see them, but when I go out into the parks, I forget about the performer and see the character, even after over a year of working there. Also, it can be great fun to have character performers who actually know you. You'll have a character run up to you all excited to see you, and then you go nuts trying to figure out how they know you!
For many people, when they work at Disney all the time, they lose desire to play there anymore. I'm not that way though. There may be times when I want a break, but I always come back. I love the parks, and the freedom of free admission year round means that you can pop in just for a few hrs and leave if you so choose. You no longer feel rushed to experience as much as you can. It's wonderful.