I never use one, but I know others swear by it. I just turn down my oven to 350 and bake for 55 minutes, then when it comes out, press it down with a clean dishtowel to get rid of any "humpage" lol.
I've recently started using a flower nail, just because so many people swear by them on this site!
I have to say that I worked at a bakery for 2 years, and we NEVER used heating cores, even on 16x24" sheets. And until recently, I baked without them. (That is 5 years of baking, without using a nail/core).
Honestly, I don't really notice much of a difference. I think they might have baked a bit more level, but not more thoroughly. I used them for layers larger than 10", and all sheet cakes.
Just my humble opinion! Lindsay 
Thanks everyone. I have my 10 in in the oven now and the middle looks a little soggy. I am getting worried because I got lazy and didn't use the core. I hope it isn't raw 
The edges are starting to look too crisp for my taste and the middle isn't even close to done.I always seem to have disasters
I've never used a heating core, but have recently started using the flower nail. I also recently turned my oven down to 325 on all my cakes because the edges seemed crispy to me. I can't if one or the other is helping more since I started them both at the same time, but I think with the nail the cakes are more even, which helps with leveling.
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