What does the company do well? Optimizely is a very powerful testing tool that has great reporting data to help you make informed decisions. I enjoy the WYSIWYG editor option, however I generally just use the jQuery code editor. I love being able to make a quick change to my website and have it go live instantly while I wait for my dev team to hard-code the my changes to production.
What does the company not do well? There can be a screen "flicker" effect while an experience is loading for the user. That is to say, your original page may load, and then a second later, the Optimizely changes will come into effect. This can be mitigated somewhat depending on your .js tag implementation. Also, for some reason, the numbers I see in Optimizely don't always match up with the reporting I see in Google Analytics, forcing us to usually just look for trends rather than hard data.
What about the company do you wish you had known before purchasing? Optimizely used to be a spectacularly powerful tool at a great value. Now our Starter plan is being sunset for a much more expensive plan, which will most likely cause us to use a different software altogether, such as VWO or Monetate. My one piece of advice for anybody just getting into the website optimization space is to test every change you to your site, and make sure your tests play through all the way to significance. Sometimes pretty doesn't always win, and customers will probably love the variation that you hate the most.