I was long a user of monopods out in the field. With the advent of image stabilization lenses, I find I almost never haul it out any more. About the only use case I have for it now is when I'm using my 300/1.4x combo with waterbirds or shorebirds, and even then, I'm most likely to either carry the tripod or go handheld.
I see the use case for monopods in two broad areas: the situation above where you want the ability to reposition quickly but want the added stability, or a similar setup where you're shooting sports (baseball or football), especially extended sessions where handholding might cause arm fatigue. The third case (locations like museum where a monopod might be allowed and tripod not...) might make sense in certain special cases.
But in general, improvements in lens speed/quality and IS, and carbon fiber tripods reducing weight, have made monopods a niche tool. lighter tripods means I'm not as likely to choose the monopod as a compromise for weight, and remember you can always just put down one leg of a tripod and use it as a funny looking monopod, but you can't extend the other two legs of a monopod for stability...
In some cases the "string monopod" might be a useful alternative, too. use a bolt in the tripod connector (it's a 1/4 20, I believe) and attach a string to it that reachers the ground. then stand on the string and pull up to create tension -- and it'll stabilize the camera. I need to experiment more with this, but unless you are using the monopod to avoid arm fatigue, it seems to give you all the benefits of the monopod for stability, at almost no weight and similar stability improvements.
So for me, it's using IS lenses where possible, and either carrying a tripod or going handheld. monopods just aren't that useful with modern technology.