Turns out that I really do not like the S-n-S - at all. The individual little holders do not "firmly" hold bobbins that don't have a lot of thread on them, the threads come loose & it's a pain. But the thing that is more annoying is that the levels of the S-n-S come apart way too easily. Every time I pick it up from a layer other than the bottom, the lower layers fall apart. It's not a major big deal and I've just been living with it.
Today I bought a pack of "Bobbin Buddies," which are little pink holders, and they are fabulous (no photo here)! They hold bobbins with any amount of thread, large or small, securely, and the bobbins don't unwind. The Bobbin Buddies made me decide the Clover S-n-S product wasn't worth keeping. I decided to throw it away. But then, I had a lightbulb moment - I cut the little holders off of the stems of the Clover S-n-S and ended up with a bunch (30, to be exact) of Bobbin Buddy-like pieces! Yay - because a pack of 12 Bobbin Buddies cost nearly $7.00! I'm a happy camper again, and am glad to put the pieces of the S-n-S back to use, even if I only use them for fully-wound bobbins in the future.