The moment that sold me on these was pulling out the off season blankets from under the bed without sneezing my way through a cloud of dust, everything still flat, still smelling like nothing, zipped into its own little brick. That's the whole pitch, really. Not that it holds things, a box holds things too, but that it holds them without taking up a shelf's worth of air while it does it.
Clear is the finish to start with, and I'd argue the only one worth bothering with if you're buying your first few. You want to see what's inside without reading a label, especially six months from now when you've forgotten what you packed. Frosted or tinted ones look tidier on a shelf, sure, but they're useless the second you're searching for one specific sweater at eleven at night. Save the opaque, nice looking fabric versions for things you're not hunting through often, like spare bedding in a closet nobody else opens.
If you own none of these and want to own one, get the medium zippered kind with a sturdy handle on top, the size that fits a couple of pillows or a winter coat rotation. It's the size that does the most jobs. Too small and you're buying five of them for one closet. Too big and it's awkward to lift when it's full, which defeats the point of something meant to make storage easier, not heavier.
The difference between a good one and a bad one shows up about a year in, not on day one. Cheap ones have zippers that start catching on nothing, then catching on everything, until you're wrestling a bag shut with your knee. A well made one has a zipper that still glides after a move, seams that are actually sewn and reinforced rather than just heat sealed, and a base sturdy enough that it doesn't sag into a sad little puddle shape when it's sitting on a closet floor for months. Press a seam between your fingers before you buy. If it feels like a sandwich bag, it'll act like one.
These earn their keep most in homes with real seasonal swings, where you're swapping wardrobes twice a year, or anywhere storage space is split between a closet and somewhere less convenient, a garage, an attic, under a bed frame. Humid climates are where the good ones really prove themselves, keeping damp and must out of things that would otherwise come back smelling like a basement.
They won't fix a closet that's too small. What they do is give you a room back half of its floor twice a year, the half you forgot was under all that out of season stuff in the first place.