I use the Fermtech wine thief exclusively. It's easy to use, and obviates the need for cylinders to float the hydrometer in, and some arrangement to siphon or suck the beer out. The original idea of the "thief" is just a tube open at both ends. It's lowered into the liquid (which can be beer, wine, whisky, etc.), then a thumb is placed over the top end, vacuum holds the liquid in the tube, it's raised out and the sample is tested / tasted, etc.
The Fermtech thief has a simple one-way valve at the bottom, and has an opening at the top large enough to accommodate the hydrometer that came with my kit. It's simplicity itself to lower the thief into a plastic bucket fermenter, get a sample, then give the hydro a twirl to dislodge any bubbles, and take a reading. No thumb action is required, because the one-way valve at the bottom keeps the liquid in the thief.
Carboys, for me, usually require a slightly different technique, which I observed on some YouTube video. If the liquid is far enough up in the neck, immersing the thief far enough down to get sufficient liquid will overflow the carboy somewhat. If the liquid is a bit low, it may be impossible to lower the thief far enough to get enough liquid. The technique is to lower it into the liquid as far as possible, and get more liquid into the thief by rapidly "jerking" the thief up & down in the wort / beer. As the thief goes down, a bit more beer goes through the one-way valve, but can't escape. When enough liquid fills the thief, take the reading.
I read a thread on another forum in which someone said that their hydro wouldn't float straight up in their thief, so that it rested against the side, making accurate readings impossible. Mine works fine, but it is true that the diameter of the thief is not as large as most cylinders I've seen for sale, which is another simple way to float the hydrometer.
In summary, I'm a big believer in the Fermtech wine thief, and I also really like their autosiphon and bottling wand. Durable, easy to use, and simple to clean. Of course, any such gadget should be thoroughly rinsed immediately after use, to prevent any wort / beer from drying on it, and then sanitized before use. I employ a 30" wallpaper tray that I bought at the hardware store to soak all the "gadgets" an fittings. It's great for things like hoses, the thief, and the autosiphon.