Product review (5 stars): Replacement Parts and a low CFM adapter
As others have mentioned this is a great product.  But no matter how well you coat it, eventually you will need a new cap for the unit...it's just the nature of plastic to break down over time and turn brittle.

Fortunately, the folks at Heartland sell replacements.  They are super nice and can be reached at 888-772-2345.  Caps are just $5.75 (as of November 2011).

They also have a simple spring kit that adapts the unit for low CFM applications like a bathroom vent.  The kit also sells for $5.75 and installs in less than 5 minutes.