Cheap and Adds some spice to any situation.
I got these on a whim because of how cheap they were and realised I really like them. You (or a partner) is in control of the level of suction and it can go from a bit of added sensation to light pain play. Very versatile and can be used in many contexts.
The plungers seal well and do not leak air over time, and the plastic seems sturdy. I would recommend these to anyone who enjoys nipple play
The suction on these is amazing for their price, they stay in place really well with a little lubrication around the rim. I even tried one as a clit sucker, and holy WOW. It's my new favourite thing, can hardly believe something so simple has made such a difference to my play time. My nipples and clit are insanely sensitive after using them, and I generally leave them on for about 15 minutes while I warm up.
They're pretty easy to take apart and clean, I'd even clean them before use if I got them again because small threads of plastic come out near the thread (above the suction but inside the plastic cup) with the first few screws up&down, they're fine after they're worn in though and the screw mechanism gets smoother the more you use them.
Just get them, don't think twice about it. They're easily maintained and so versatile, you'll thank yourself later! Would definitely recommend going slow if you're testing limits with it though, hard and fast just makes you bruise. I'd particularly recommend these for anyone who likes nipple play, or just generally getting creative in making erogenous zones more sensitive.