I have always used Marshmallow fondant and find it to be quick, easy and super cheap... not to mention fantastic tasting.
First off... I recommend The Mat or a sheeter. The Mat is really nice because you use veg shortening instead of confectioners sugar and you don't dry your fondant out.
Second, make sure you are not using the wrong colors. The first few times I made the MMF I had it completely sieze up on me because I was using dropping red color in a pinch. Never, ever, ever. Just like with chocolate, using the wrong colors will stiffen it up and there is no fixing it after that. For deep colors, I use the modeling chocolate mixed with fondant and powdered color technique to avoid problems.
And, lastly... how I make my MMF:
I use the 10 oz package of marshmallows in a well greased bowl, that is microwave safe. I take a cupped hand and a 2 or 3 handfuls of water. Then I microwave it for 30 seconds stirring in between until it's all melted. I add 4 cups of con. sugar and stir in the bowl until it's mostly stirred in. I then grease up the counter and start to kneed it. If it's too wet still, I add handfuls of sugar one at a time and kneed them in. Then grease up well again and give it a few more kneeds.
Oddly enough, I actually like to use it right after making it. I find it just works better that way. I mean, I can use it later, but it's usually absolutely perfect right then, so I make it right as I need it. :)
Hope this helps.