The thing I reach for most mornings isn't the fancy base layer, it's the lightweight top that's basically a long sleeved T-shirt with a bit more stretch, the one I can walk the dog in, do a yoga class in, then sit at a desk in without anyone clocking I've not changed. If your week has any mix of movement and ordinary life in it, that in between weight is what earns its wear, not the thick compression stuff that only makes sense for an actual session.
Where it goes wrong is pairing it with anything heavy. A thick fleece layer on top defeats the point, you overheat in minutes because the fabric's whole job is to breathe and wick, and smothering it stops that working. Same goes for wearing it under a structured sports bra with wide bands, the thin fabric shows every seam and clasp through it, which looks more like underwear than activewear. It's also not the one for genuinely cold outdoor sessions or anything with real impact, the fabric's too thin to hold you in properly and you'll feel every bit of bounce it doesn't manage.
On sizing, the main thing to watch is how it behaves once it's warm and slightly damp, because lightweight knits stretch more than they look like they will on the hanger. Buy it true to size rather than sizing down for a snug look, since a tight fit in the shop often means see through across the chest and back within a few wears. Check the length sitting down as well as standing, a hem that looks fine standing can ride up over the lower back the second you bend forward, which is the complaint I hear most and the one that's easiest to avoid by checking before you buy.
What tells a good one from a disappointing one shows up after a few washes, not on day one. Cheap lightweight fabric goes thin and slightly see through at the seat and underarms within a season, and the seams start to pucker. A better made one keeps its shrug and recovery, the fabric springs back rather than sagging, flat seams stay flat instead of rolling, and any drawstring or zip pull is properly finished rather than fraying at the first tug. Thrown in a bag after a class and left there overnight, the good ones come out barely creased, which tells you more about the quality than anything on the label does.
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