What do you think about A/B testing?

More and more Email marketing solutions are integrated with the A B testing feature, also known as split testing, which usually helps you to send out two email campaigns to part of target recipients and pick out the one that got better response or conversion rate and send it to the rest target recipients.
I personally think that it is a brilliant feature. But I would like to know your opinions or your experience of using this feature. Because my company provides email marketing tool named KoobooMail and we are looking forward for ideas to improve ourselves.
I would be grateful if you can generously share your ideas.

See original post by sue517