All-Clad makes great products. I previously had the D3 and later the Copper Core and they are certainly good products. However, I would argue that the Demeyere is one of the pinnacles of multi-ply cookware for several reasons:

First, the amount of cladding used is greater than most other multi-ply cookware. The Proline skillets use twice the cladding materials as the D3, for example, giving it exceptionally even heating, exceptional sidewall heating, and exceptional heat retention approaching the performance of 4-5mm cast iron.

Second, they use welded handles instead of rivets, which makes cleanup easier, especially with fry pans and saute pans where one is cooking with a lot of oils.

Third, the electro-chemical surface treatment they use makes their pans perform better than other stainless cookware I have used with respect to ease in obtaining nonstick performance and ease of cleaning.

Fourth, the handle design is subjectively more ergonomic, and objectively better at staying cool over prolonged periods of cooking compared to my D3/CC.

Fifth, the optional oven-safe borosilicate glass lids that compliment the stainless lids are exceptional quality and add versatility. Having them for fry pans is especially nice.

Sixth, their sealed edges on their fully clad fry pans and sauciers greatly reduces the corrosion risk that open-edged multi ply cookware are vulnerable to over many years of use.

Seventh, the Atlantis/Proline has better warp resistance than most other stainless cookware, including the Copper Core and D3, and is an especially good choice for use on induction, which can be extremely hard on cookware.

All-Clad makes great products, but saying it is, "the only cookware worth buying in that category" is really not a fair assessment of the many great multi-ply cookware options available.