It depends totally on what type of photography I intend doing.

I invariably use one for landscape shots,  I never use one for general walkabout photography when I am looking for spontaneous shots around town. 
I always keep one in the car in case it is needed.

I have never really got on with monopods, except as a handy stick - for walking, or holding back brambles, etc.

And every one said, 'If we only live,
We too will go to sea in a Sieve,---
To the hills of the Chankly Bore!'
Far and few, far and few, Are the lands where the Jumblies live;
Their heads are green, and their hands are blue, And they went to sea in a Sieve.

Edward Lear
http://www.nonsenselit.org/Lear/ns/jumblies.html