[scott_w]

I've worked in situations where A/B testing is heavily used (and useful) and in situations where it was completely useless. Both in the same company but at separate points in the flow.

Where you have enough users (maybe you're a major online retailer), A/B testing should be a vital part of your toolkit. Not the only tool, but you definitely need to test every change you make. If you can gather enough data within 24 hours, why wouldn't you test your change?

That being said, A/B testing isn't the be-all and end-all. It just gives you some information to make a decision. You still need to know your customer, speak to them, survey, observe, etc. You might even pick a "losing" variation with the aim being to reach a more optimal business outcome. Data doesn't give you the right to abdicate your responsibility to make good decisions.

There are cases where A/B testing can't help at all. A great example is in low-volume but critical flows (think SaaS conversion funnels). For these, you need to rely on the other skills you have at your disposal.