I have gotten a lot of mileage out of Fifth Edition Foes. It provides a very different sent of monsters from what the (excellent) Monster Manual provides, which makes it good for filling out those weird, pulpy adventures, or just giving you a variant type of monster when you want to surprise your players with something they've never seen before. I've managed to get 6 or 7 into direct confrontation with my player characters so far, and I have a bunch of others who have just been waiting in the wings.
Some of my best "oh ****" moments have come from monsters from this book, especially because 2 of my players also GM and are pretty familiar with the contents of the Monster Manual. One of my players will even "remind" me of features that I am forgetting or mis-reading. I can still put the fear of God into them with a room-full of wraiths, but there's something great about that first time you see a new monster and realize, "oh my God, it does that!? run away!"
Quests of Doom seems like a solid product and is definitely great bang for your buck. If you're interested in short adventures a la dungeon magazine, grabbing Vol 1 is a bit like picking up 3 or 4 issues of Dungeon. On the other hand, I've only run one adventure so far (The Noble Rot) and found it to be only so-so at our table. I've written a somewhat tedious account of it on this forum.
We might have a wizard joining our group soon, and I've thought about picking up Lost Spells to use as treasure items to be found.