These are pretty common under sinks and for toilets here. I can't say I've encountered a sink or toilet without them.

The older ones are all gate valves; they leak if you close them and open them again, much like a regular seldom-used gate valve would do. I don't trust them.

To avoid hassle I turn off the nearest inline ball valve. If there is a threaded gate valve at the fixture, I'll replace it with a ball valve style. Generally if I'm replacing some at-fixture plumbing I'm not interested in having to sweat anything to get the job done.

Every time I shut off a gate valve it ends up leaking when I turn it back on and I have to replace it. I have an couple extra ball valves laying around in case I decide I'm a fucking idiot and turn one of the gate valves off.