Knee high boots suit anyone who wants trousers and dresses to stop being a separate decision every morning, which is most of us. They work best if you've got a calf that falls somewhere in the ordinary range for your size, because fit is where this whole category lives or dies. If your calves are on the fuller side, think twice before buying from a rail with no width options at all, you'll get them halfway up and stop, and that's a miserable way to spend a Saturday. Narrow calves have the opposite problem, gaping at the top like a wellington, so it's worth checking for an adjustable strap or elastic gusset before you commit.
Once you've got a pair that actually fits, they do a lot of quiet lifting in an ordinary wardrobe. They sit well under straight leg jeans tucked in, under a midi skirt with the hem skimming the top of the boot, and under tights on the days a dress needs covering up without reaching for a coat that's too warm for the weather. What separates a boot that lasts from one that's sad by November is mostly in the parts you can check with your hands before you buy. Bend the sole in the shop, a good one flexes at the ball of the foot and springs back rather than creasing into a permanent fold. Run a finger round the top edge, real leather or a decent leather look has a bit of weight and a slight give, cheap stuff feels thin and almost papery and will crack at the fold within a season. Check the zip pull, metal rather than plastic, and tug it halfway up to see if it catches, because a zip that sticks in the shop will be unusable after six months of rain. The stitching round the sole should be even and close, gaps there are where water gets in first.
If you've never owned a pair and want one to start with, go for a plain black boot with a block heel rather than a stiletto, a low block is far more forgiving for walking any real distance and still looks sharp with nearly everything. Skip the elaborate buckles and studs for your first pair, plain leather or a good faux takes you further because it doesn't date and it doesn't fight with prints or patterns the way hardware can.
Next time you're looking at a pair, check the calf fit and the zip before anything else, the rest you can live with.