I haven’t worn one myself, but my cousin with one has been quite happy with it, and it looks very natural on her. She wore a full wig for years, and raves about how comfortable and natural looking this is in comparison, and she used to sell and style wigs, so she knows her stuff. Her hair is wavy, between chin and shoulder length, and her ‘kippa sheitel’ (as they’re termed in my circles) is similar; I don’t know how it would work if her actual hair was significantly shorter/longer.
I’m an Orthodox Jewish woman and have just gotten married, so I now cover my hair. I have a 20’ wavy-curly fall, which means that it’s pretty heavy as these things go, and it stays on pretty solidly - I wouldn’t be worried. I have long hair that I need to fit up under my fall, which I couldn’t do without a full cap, as I’d have nowhere to put it. (My fall actually comes with a small, pouchlike extension to the cap that sits at the top of my neck, so that I can stuff my braid up into it fairly securely.) I don’t know where you’d put long hair underneath a kipa sheitel without giant lumps forming.
You’d definitely need to go shopping in person, to get a good color match, if you want any of your own hair showing at all. My fall was custom-made for me (long curly human hair ones are definitely best done custom, although for straight it’s probably a better idea to go semi-custom) so color matching wasn’t an issue, but it needs to be very close for anything other than a full wig. Even the normal oxidizing that happens with time (anybody with long hair can see this - hold the bottom of your hair up by the top, and you’ll probably see that it’s a shade or two paler/redder than when it first grew out of your head) gets to be a problem with falls, as you can start to tell the difference between the woman’s actual hair and the fall. (If you do buy one, and you’re deciding between two shades, get the slightly darker one, as when it fades, it’ll end up being closer to your hair than the paler one, which will get too light.)
I’m new enough at wearing mine that my friends have made gentle fun of the way that I still seem stiff in it, like I’m afraid it’s about to fall off (I’ve been married less than a month, and don’t wear it most days, so I’ve only worn it 6-7 times) but it actually stays on fairly securely, to the point that I actually have trouble getting the clips unsnapped when I want to get it off! They’re really fairly secure; my major comfort issue with the thing is that it gives me a headache after a couple of hours, but I’m not worried that it’s about to fall off. If I wear it for many hours running (the longest I’ve ever gone is about seven hours, to a wedding), it does slide back on my head very slightly, probably ending up about a quarter-inch behind where I originally set it on my head, but the clips won’t come unsnapped without significant pressure at a specific angle, which isn’t going to happen by accident.