The first thing I do with a glass ornament is hold it up to a window, not the shop light. That's where you see the actual color, whether it's got that cheap flat tint or something with a bit of depth to it, bubbles, swirls, a hand blown wobble that tells you a person made this and not a mold on a factory line somewhere. Weight matters too. A good one has a little heft in your palm, not that hollow rattle that means it's going to crack on its first bump against a branch.
Where I wouldn't put these is anywhere low on a tree with small kids or a cat that treats ornaments as personal projectiles. I learned that one the hard way with a set of mercury glass baubles that lasted exactly one Christmas before they became a pile of silver dust on the hallway rug. They also don't love a mantel with a lot of foot traffic nearby, or a shelf near a door that gets slammed. Glass doesn't forgive carelessness, it just waits for the one wrong moment.
They do age well though, which is more than I can say for a lot of decorations. The color doesn't fade the way painted plastic ones go chalky after a few years in a hot attic. What changes is the finish, mercury glass especially will mellow and develop tiny flecks and cloudiness inside that actually look better with time, like it's earning some history. Store them upright in their own compartments if you can, newspaper or tissue if you can't, and never just toss them loose in a bin with other stuff. A cardboard box with dividers from a craft store costs almost nothing and saves you a box of disappointment every January.
In an ordinary room, glass ornaments sit nicely against anything with a bit of texture to offset the shine, a chunky knit stocking, a wood mantel, dried branches in a jar. They look wrong stacked next to a lot of other shiny things, too much glitter and glass together just reads busy and a little desperate. One good cluster on a shelf or a few hung in a window on ribbon does more than a tree full of them competing for attention.
If you're buying a set without seeing them in person, check the listed diameter against something you already own, a tennis ball is close to a small ornament, a grapefruit is close to a large one, and that tells you more than any product photo will.