Generalized affine Grassmannian slices are certain algebraic varieties, which arise as Coulomb branches for 3d N=4 quiver gauge theories as shown by Braverman-Finkelberg-Nakajima. They are natural generalizations of usual affine Grassmannian slices and so have many expected geometric properties, but not all of these are fully understood. I'll discuss recent work with Dinakar Muthiah, which shows that "open" generalized slices are smooth. As a consequence this establishes decompositions into symplectic leaves, as conjectured by Braverman-Finkelberg-Nakajima.