Dan-O is the only one professional enough...
I use them and I really appreciate it when people around me use them. Part of being at a range is not only checking yourself, but also double checking those around you. The chamber flag makes double checking others guns extremely easy. You can look up and down a line and KNOW that nothing is in the chamber and that there is a physical impediment to anything entering the chamber. They eliminate forgetting to clear the chamber of accidentally dropping the slide and chambering a round. They are dirt cheap and you can even get them for free some of places. The CMP club I shoot at hands them out like candy on Halloween. I wish more guns came with them instead of trigger locks that get thrown away. I don't have brake lights to make sure I know when I am braking.
I use them at a public range and when transporting to the range. I leave the flags in my competition guns so I don't forget them.
When reading above, consider I am a person who does not look favorably upon manual safeties. On many of my guns I simply do not use them.