A good gaming supplement expands the game; it provides some new and interesting spells, items, few cool plot points, maybe some adventure hooks. A great gaming supplement changes the game in exciting ways that make you want to dive into the world. Solve et Coagula, from the first paragraph of the first page, is a great supplement; while still remaining absolutley true to the lore and feel of the world, this short supplement adds a whole new axis of play to the game, inviting whole new stories revolving around and expertly weaving in other elements of the game (such as the "lifestyle" rules) that, while otherwise mechancially necessary, didn't really feel like an integral part of the game until this connected the dots.
There's more, of course, but this book is only 32 pages long, and I don't want to give away the farm. If you find yourself curious, even a little, about the material in this book, do yourself a favor and pick it up. You won't regret it.