Faucet spout perlator: worth it?
I've had a couple of these over the years and found that their biggest benefit was the ability to convert to a spray for better rinsing of dishes if your sink doesn't have a separate sprayer, not so much their water-saving ability.  Even though a 40% reduction sounds like a lot, your kitchen sink doesn't use that much water to begin with compared to, say, your shower or clothes washer.  So reducing your kitchen sink usage by 40% may not produce noticeable results on your water bill.