I own 3 monopods (1 Gitzo and 2 Manfrotto)so I guess I’m with the guys that find them useful (I plan to sell one soon though) - I should add that I also own 4 tripods... (2 video and 2 still)
If you do photography that requires one or more of the following three things then a monopod would be useful
1) You use a heavy lens but a tripod is not practical {the monopod will save your arms}
2) You take repetitive photography - think cars around a track, runners at a finish line, athletics, {the monopod will help to make the shots consistent}
3) You need a slow Sutter speed but a tripod is not practical {climb a mountain, a long walk, at a stadium}
I would suggest if you are considering buying a monopod buy a tall solid tough one and if you are considering using it for wildlife buy Gitzo with the twist locks as they will not scare the wildlife as much. The noise that Manfrotto "snap to lock" type create will spoil some of your photos (due to the noise) but they are quicker to set up an break down then Gitzo