Friendly plastic is easy to use, and it gives you the ability to mold precisely what you want as long as the plastic has some type of base to sit on top of.
Friendly plastic is also messy, flimsy when stretched out, and it always dries in white. If you're willing to learn how to use it though, it can create some great effects.
Friendly plastic is an item that's fairly uncommon except in the most competitive cosplay circles and the most serious costume contests. Friendly plastic is a plastic material that comes in small pellets that can be heated up (they go from white to clear when they're hot enough), and then molded into whatever shape you need them to be. As the plastic cools back down again it hardens into white plastic that's firm and strong. Friendly plastic gives you the ability to create whatever shape you happen to need, no matter how strange or unusual it might be. And what's best about this material is that, if you heat it up again then it will melt again so that you can use it over and over.
However, as with any great boon friendly plastic does have a few disadvantages that hopeful costumers and costumer role players should be aware of. First off, it can only be used hot. This means that you have a very short period of time to make use of the friendly plastic, and while you're molding it then the plastic might be hot enough to burn your fingertips. It also has a tendency of making whatever bowl or cup you're putting it in (I recommend heating the pellets up in water) useless for anything but melting friendly plastic. Ditto the spoon or other utensil you scoop it out with. This means that you have a short time to mold the plastic into the shape that you want, and that you should be careful that it's just how you want it to be before the friendly plastic hardens and you'd need to start all over again.
As a costumer who's used friendly plastic, it works best when you're applying it to a base material. For instance, say you want to add something to a plastic mask like a strange fringe, horns or something else. Warm up some friendly plastic and then shape it directly on the base. If you want to use friendly plastic for your cosplay sword or your Halloween costume accessories though, beware that the stuff is fairly flimsy when stretched out and not given a core of support. If you're patient enough to use friendly plastic though, and you keep at it till you've got just what you want then you'll have a costume that you may want to wear for years, and which might win you a few awards as well.
All in all, friendly plastic is more of a boon than a curse for costumers and the costumes they make. It may be a little pricey and hard to find, but friendly plastic is a godsend when you're trying to mimic an effect that there just aren't any props or accessories for.