I've never used one becuase primarily I never wanted to put a big 'ole hole in the middle of my cake. Second, I've never had a need to use one.
I grease-only-no-flour my pans (haven't floured pans in 30 years), use the baking strips and reduce the oven temp to 325 (in a regular home oven .... might need adjsted for convection or comm'l ovens).
Here's a thread where I explain the science behind how baking strips work: http://cakecentral.com/cake-decorating-ftopicp-6626888.html#6626888
..moving to Decorating forum. 
Everything you need to know to make, decorate and assemble tiered/stacked/layer cakes:
http://www.cakecentral.com/cake-decorating-ftopict-605188.html
Relax, get a glass/cup of your favorite beverage because this a long read.
HTH
The biggest layer I've done is a 16" square and I didn't use a heating core. What I did use was baking strips to keep the outside from baking before the center did.
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