I previously used Optimizely to conduct A/B tests for our conversion landing pages. We needed to increase conversions and Optimizely has a number of features that allowed us to determine what worked best. I also used Optimizely on our main website to test out certain scenarios regarding our customer service activities.
Optimizely is an all-in once service compared to the others I tested. Visual Website Optimizer (VWO) is very solid and probably a close second as far as I'm concerned. Their servicing was a little lacking which made it hard to ramp up the testing. Unbounce is very user-friendly however doesn't allow for an experienced coder to customize test as well.
For what it does Optimizely is very good. Although I did have to splurge and pay for the larger package to get all of the bells and whistles I was looking for, I was able to expand my testing scenarios. Looking at the landscape of all other testing services Optimizely is at the top.