AHow hard is it to use a stincile on the side of a fondant cake? It looks pretty easy but then I think the stencile would move and I would ruin my cake :-( I love the look of it but I don't want to spend $20+ on one and not be able to use it. Thanks!

Not hard at all. I've done it.  Takes a bit of practice.  I suggest you put some fondant on the side of something like a cake pan, let it dry a bit and then apply.  Keeping it still is the hardest part.   Here is a link that can help you.  

Make sure you're sides are completely smooth as well.  Bumps gave me hearthburn!  Other than that, it was simple.

AThanks everyone! I ordered one to try :-)

AThis was very helpful! It even gave me an idea for the wedding cake I have coming up! Thanks again a bunch. Wish me luck :-)

You'll do great.  Not hard at all.

I use masking tape to keep the stencil still , it is like a couple of extra pairs of hands.