Product review (4 stars): Water Filters =  Printer Cartridge / Toner Situation?
I read all the threads below before I made my Sentinel purchase of this filter (for less than  the cost of GE Direct). I'm no scientist nor water purity expert, but my guess is that both filters have carbon/charcoal core that are similar and filter the water similarly.

To remind me to purchase a filter for my GE Fridge, I got a mailer via U.S. Mail from GE that offers "Water Filter Savings Direct from GE" with a special discount CODE.  I called the 800#.  After the "discount" the pricing is about the same as here on Amazon btw.... (They did offer free shipping; I have Amazon Prime which provided for free shipping).
For the above promotion, GE sent me a color 1-page brochure with the "Water Filter Savings" special.  A color 1-pager isn't cheap to produce and mass mail.  Also, the associated 1-800# staffed by contact center agents adds a bit more cost.  It's very unlikely that GE would have a Direct Mail campaign for a water filter if there wasn't a a high profit (over priced) item.

What I think we have is that the GE Filter is the equivalent of buying Printer Toner or Printer Cartridges at very high price markup from the printer manufacturers.  GE wouldn't be able to derive strong ROI from their direct marketing if the water filter wasn't a very high margin product.

My guess is Sentinel is providing a like product at lower cost.  (And btw, I'm a marketing guy in Atlanta, never heard of Sentinel before my purchase, not affiliated in any way)