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Key to this success is our customers or
Key to this success are our customers ?
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I'd agree - even if I'd prefer (no real reason why)
"Key to our success is our customers".
Comments  
I guess both are.

(the) key to this success is our customers.

In this case "the key" is the subject, so it is "the key is (our customers)". Grammatically correct. On the other hand, take a look at this sentence:

our customers are the key to this success.

can be re-written as:

(the) key to this success are our customers.

so we are facing a typical case of inversion, which is also acceptable.

Hope this helps! Emotion: smile
 Pemmican's reply was promoted to an answer.