Quests of Doom is excellent for my purposes. (There are some adventures I'll use for my game, but my main purposes are idea-mining and giving myself solo adventures to play through on my own, so I can have surprises.) 90% of the adventures in it are solid, believable and usable, and the editing conventions like in-line stats also make the product easy to use.
Fifth Edition Foes is great. The monsters are more interesting than MM monsters; probably more difficult too but that's good IMO, and they still obey DMG rules on CR. I unfortunately haven't used the monsters much yet in play, but that's because referencing a laptop during play is awkward. I'm sure I'll use it more once the hardcovers arrive.
Book of Lost Spells is fantastic. Doubles or triples (?) the number of spells potentially in play, forcing casters into some really painful dilemmas. There are a few spells which are overpowered (Iron Core) and a few which are underpowered and completely dominated by PHB spells (Aura of Nobility), but the main thing this adds is VARIETY. It feels like what the Tome of Magic did for priests in AD&D2: made them interesting for lots of things besides healing, but at an opportunity cost. I'm definitely going to use this book's undead-enhancement spells like Iron Body against my players. Not that I couldn't have arbitrarily enhanced the undead opposition anyway, but I dislike doing that unless it was either a one-time fluke like a negative energy surge or I know how the PCs could theoretically achieve the same thing. In this case it's the latter.