They work pretty well for what they are.
They aren’t as durable as a loop to loop addition sink tip or standard sinking line, but they do help flies get down deeper for some streamer fishing scenarios. Only down side if when you get a fly hung and have to pull to snap it you never know where it will snap from, sometimes it’s right at the end and sometimes it’s 2-3’ up. So your 9’ leader turns to 6-7’ pretty quick.
I like them a lot for 2-4wt’s where wanting a sink tip for things like white bass fishing in the spring. It essentially turn yours normal floating into a sink tip once you dial in what weight your small rod likes.