I used mixed in key for about eight months. That was about six years ago. It's an excellent learning tool, but you really need to just use your ears. Learn what lessons you can from it, and ditch it. Like the other posters said, it has its limitations. You as a human being, have very few of those limitations.

Edit: your appeal to authority, as to why some of the biggest DJs in the business use it, is invalid. Some of the best DJs to have ever lived completed their careers before mixed in key was invented. It's just another tool, not manna from heaven