Product review (3 stars): I've switched to another brand now...
I have bought these in the past and they worked pretty well. My water isn't too hard, but I was using RO filtered water in my ultrasonic units too. This year, I went to buy more cartridges and priced them online here and on other sites. BB&B is where I normally buy them though, as I have tons of coupons for that store; so; it always worked out that buying them at my local BB&B was the best deal, and I didn't have to wait for them.

This time when I went to buy new filters, I noticed BB&B isn't selling Venta Sonic Ultrasonic humidifiers in the store anymore, though they still had some of the filters on the shelf, but they were from last year, and they're priced at $14.99. The store manager wouldn't let me use my $5 off coupons because of this. She said I could use my 20% off coupons instead. Working the numbers though showed that if I switched to the Air-O-Swiss ones at $15.99 each, I could use my $5 off coupon for each filter and saved about $1 per each, over buying the old Venta ones with a 20% off coupon. Just do the math!