It's not even a little yellow box now. Fairly recently they are now battery powered tiny clips over your finger with a built in display just big enough to show pulse and oxygen levels.

Apparently they are cheap enough and so easy to use that nurses are buying their own and carrying them around along with their stethoscopes.

The only concern, like with stethoscopes, is they are likely to be the dirtiest instruments in the room and they are so quick and easy to clip on a finger it's easy to forget to wipe them down before and after each use.

To try to cut down on this germ vector, they also make a disposable version of the sensor that goes on your finger that can be left with each patient instead of traveling with the monitor.