Stand a cheap geometric rug next to a good one and the difference hits you before you even register the pattern. The bad one has lines that wobble slightly out of true, colors that look printed on top of the fiber instead of woven through it, and edges that curl up within a month no matter how flat you steam it. The good one has crisp geometry, a weight to it that keeps it flat against the floor, and color that goes all the way down into the pile so a little wear doesn't turn it patchy.
If you own zero patterned rugs right now, start with something small scale and two toned rather than a loud, multicolor burst. A simple diamond or chevron in two close shades is easier to live with long term than a rug that was clearly chosen to make a statement, and it gives you room to add more pattern elsewhere later without the floor fighting the walls. On sizing, measure the actual seating area, not the room. In a living room, the front legs of your sofa and chairs should sit on the rug, which usually means you need something bigger than instinct tells you, closer to eight by ten than five by seven for a normal sized room. In a bedroom, go wide enough that it extends past both sides of the bed so your feet land on it getting up, not on cold floor. The common mistake is buying a rug sized like a bath mat for a space that needed a proper anchor, and the second most common one is picking a pattern so busy it reads as clutter the second you put a patterned cushion or a striped throw anywhere near it. Keep the rest of the room calmer than the rug. Solid furniture, plain or textured cushions, maybe one other pattern at most and in a totally different scale, works far better than trying to match everything.
These rugs do their best work in living rooms and entryways where you want a bit of structure and energy at floor level, and in homes with mostly neutral furniture that could use one confident thing in the mix. They suit a rented apartment just as well as a house, since a bold floor piece can do a lot of the personality lifting when you're not allowed to touch the walls. They read a little crisper in spring and summer and a little more grounded in cooler months if the tones lean warm rather than stark black and white.
A good geometric rug is one of the few patterned things in a house that never actually goes out of fashion, it just goes in and out of your own patience for it.