I've used a 2009 White MacBook, a mid-2010 15" MBP and a mid-2012 15" rMBP. In my opinion, the White MacBook and cMBP keyboards were almost the same. After being worn out a bit, both keyboards become soft and nice to use. As for the rMBP, as said above, the key travel is much shorter. The feedback is better defined, but the overall experience just isn't as good as that on the cMBP, especially if you're used to touch typing relying on feedback.
The best keyboard, however, is on the Apple Wireless Keyboard. It has a long key travel like the cMBP and the best mechanical keyboard-like feedback.
Regards
Abhinav
UPDATE: To stay on topic, I have, on various occasions, used MBAs (all the way from the original 2008 model to the 2012 model). The keyboard isn't much different than the one on rMBPs. Key stroke is similar. Feedback is a bit lighter, though.