Hi MrM1,
Yes, you will find some percentage of feedback about soft spots or sagging (virtual or visible impressions) with every mattress … especially in this budget range. In the case of the Ultimate Dreams latex (which I assume is the mattress you mean) the percentage will be very low compared to mattresses you will usually find in this budget range because talalay latex is such a durable material (and not normally found in mattresses in this price range) and holds up better in a comfort layer than any other foam material. Most of these reports will either be because of an initial choice that was too soft in the first place, higher body weights that would do better with a firmer mattress or a higher density base layer, or in a very small percentage of cases actual defects in the foam that was supplied by the foam manufacturer (which will usually show up early in the life of the mattress). There is more about all the factors that can affect the durability and useful life of a mattress relative to different people and body types in post #2 here and the posts it links to.
Overall … for those where durability is an issue … latex is the most durable of all the foam materials used in comfort layers regardless of the name of the mattress or the manufacturer that uses it in their mattress.
Phoenix