Product review (4 stars): Not the most cost effective, but the easiest and best solution in the industry
As someone who totally flakes on filling my humidifiers regularly, these are perfect.  When used in a properly seasoned humidor, these will maintain a very accurate (+/- 2% in my experience) humidity level in your box.  It is easy to tell when they need to be replaced, as they dry up and become noticeably smaller.  Average sized humidors will need 2 to 4 packs, depending on how many cigars you have in there at the time.

These are not the most economical way to maintain even humidity levels in a humidor, far from it.  They are, in my opinion, the absolute easiest product to use among the competition.  If you are storing $100's or $1,000's worth of cigars, the price shouldn't be an issue, as the possibility of replacing quality smokes is reason enough to give them a shot.