I gotta fuck with some stuff briefly on my monk. I slurped up FD proc, Convention of Elements proc, and Spirit Guards. So it takes 5 death's breath, the item (it is consumed upon use), and 1 of each material that is acquired via cache's from each act. If you do each act your cache gets 4 (at T7 at least) of each mat per act, BUT, if you do the acts in "bonus" order you get an additional cache so you can get 8 of each item per full clear of a game. It's buggy so last night I only got 1 of my 5 extra caches.
Once you use the recipe (5 breath, 1 act I/II/III/IV/V component, legendary) you consume the legendary but add it to your library of procs you have saved. So you could ideally just eat one of every item and have your full library to do as you like. The proc effect stolen will always be max effect such as Convention of Elements is 200% elemental damage when applied. There's a list of 80ish legendaries for each slot you can steal from. I think in terms of weapons some classes will be stuck (FD for monks), but armor and jewelry may truly be a taste thing. BlizzCon ring vs Elemental Immune necks or shit like Crudest Boots vs Spirit Guards vs Gundgo's vs Cindercoat. Lotta options. Looking forward to it.