They're a must - I replaced the AC in my unit with one when I bought it. I recently ordered a second and installed it in the bathroom.
One complaint: They changed their motor supplier. My old fan has a tallish, can-shaped motor that makes a pleasant low pitch hum when running. The new fan has a much smaller, flattish motor that makes a higher-pitched whine when running.
Both work wonderfully - I just wish they hadn't changed motor styles.