The question I get most, usually from a guest standing in my kitchen eyeing the table, is whether side chairs need to match. They don't, and honestly the ones that don't match are usually the more interesting room. What matters more than matching is the size of the thing, because a side chair that looks dainty in a showroom can turn into a knee sized hazard the minute you push it under a farmhouse table built for people who do actual work with their hands.
A good side chair earns its spot in the rooms that ask people to sit for a while but not forever, a dining table, a desk, a spot by the window where someone reads with coffee. In a small kitchen, two or three at a table do what a bulky upholstered chair never could, they tuck in flush and leave you a path to the stove. In a bedroom, one by the window with a throw over the back is about the coziest thing you can do for a corner that otherwise does nothing. Where they struggle is anywhere you actually want to sink in and stay a while, a side chair with a hard wood seat is a fine place to eat a meal and a miserable place to watch three hours of a movie, and I learned that the hard way putting two in my front room thinking they'd double as a reading nook. Tom still brings up how fast our guests abandoned them for the couch.
When you're looking at one in a shop or standing in the driveway while the delivery truck idles, check the seat height against your table, not against the chair next to it, because an inch off and you'll be eating dinner with your elbows at your ears. Run a hand along the underside of the seat for rough edges, give the frame a gentle twist to feel for wobble, and if it's upholstered, press down and listen, a good one doesn't creak at you like it's filing a complaint. Wood ones should feel heavier than you expect, the light ones tend to be the first to go when six people lean back at once during Thanksgiving, and I speak from a chair related incident I won't fully relive here.
What I'd skip is buying a full set before you've sat in even one for twenty minutes, since comfort at the dinner table reveals itself slowly, not in the first minute of a showroom test sit. A single good one by a window, tried honestly, tells you more than a matched set of six ever will.