I use the Matias Quiet Pro, can't say enough good things about it. To me, the keyboard is much more user friendly then the Apple supplied keyboard. Worth the cost. Your mileage may vary.
I love my Das Keyboard. Though the new model 4 isn't yet available with a Mac key layout, the previous unit is great.
http://www.daskeyboard.com/model-s-professional-for-mac/