Our marketing team uses Optimizely to A/B test landing pages. It's a great tool to test things like buttons, taglines, images. etc. It's very easy to use and gives you really good data on how your different site variations compare. I would recommend it to any marketing staff who controls a website or landing pages.
Optimizely is a great tool for A/B testing landing pages, I'd recommend it to anyone who doesn't have a built-in A/B test workflow. I haven't come across a worthy competitor that does what Optimizely does. Their software is also much more robust than landing page platforms that have built-in A/B testing.
Optimizely is great if you use landing pages for signups. Their A/B testing will help you optimize workflows and improve your conversion rates. If you're a marketer responsible for signups, you'll want a tool like this. You can easily set up a test to see which feature converts better. Very simple to use. But if you're using something like Instapage which already has A/B testing built in, it's not worth paying for a product like Optimizely.