Personally, I struggle with using just fondant in any moulds, even silicone ones. But if you use gum paste, they're great.
I have used fondant many, mant times. The trick is to have just the right consistency. If it is too soft, you will have a heck of a time getting out in one piece. I sometimes add a smidge of tylose powder to my fondant just so it will be pliable, but will stiffen up slightly. But if I am in a hurry, I pop the moldvin the freezer for anywhere from 5 min to 10. Depending how big the mold is. Pops out every time.
Hope that helps. :o)