I tell people that I own three adult onesies because it's the official uniform when you work from home — but I'd own them even if I didn't. The best adult onesies for women are, in my opinion, a severely undervalued category of loungewear. Yes, they're like wearing a fleece blanket everywhere you go (so they're amazing for winter or for people who are always cold), but they're also surprisingly practical, too. Most have built-in slippers, hoods, and sometimes even pockets.
Adult onesies, however, are often surprisingly expensive (and that includes the poorly-made versions that won't last you longer than a season). For that reason, you want to be sure that you're getting something high-quality. Onesies have a tendency to rip at common points of stress (like under your arms or between your legs), and since the ones with built-in 'footies' are making constant contact with the floor, those wear out quickly, too. A onesie made from quality material and reinforced stitching, however, will ensure that your money is well-spent.