Sanctuary Bay and Sanctuary Belize are the same place, but with different management now (so far as anyone can tell.) I have visited it a few times and most recently took at a fairly lengthy tour with Chris Usher (son of the chairman of the board) about three months ago.
I have to say I was impressed with a good deal of what I saw and heard, though at the end of the day it's in a fairly isolated area and I think even the owners admit it will be many years before there's a real community of people and infrastructure at Sanctuary. You're around 45 minutes from Placencia village or Dangriga town by road. I feel the same way about, say, Orchid Bay in Corozal District, which is close to the same distance from Corozal Town. In both cases part of the road is dirt/gravel.
Neither place would be the place I'd want to live, a long way from medical care and amenities like a choice of restaurants, groceries and entertainment, but it clear that sizeable numbers of people have bought into the dream. In fact, Usher at Sanctuary Belize told me that on their last "tour" group of 40-some prospects who flew in from the U.S. (I assume most were couples, so it was probably 20-some couples), 100% of them agreed to buy a lot. He didn't offer any proof of that, but I have no reason to doubt him.
As to the oft-heard claims made about increases in property values, I think most economists or real estate experts would point to the difficulty in determining what the fair free market price is for lots, even resales, in a fairly remote area of Belize, with the supply and price of lots essentially controlled by the seller and released from a large inventory, and the well-known lack of verifiable real estate market data in Belize such as you have with MLS and other hard data services in places like the U.S.