For our logged in user A/B testing (often relating to email), we've built an in-house solution that uses a MOD based system to track group performance (as opposed to storing each group member in our databases).
Pretty clever implementation by the data scientist on our team.
We're currently using Google Content Experiments. The Pros are that it's incredibly easy to get a test started thanks to the presence of Google Analytics scripts in the rest of the website. The major Con is the lack of true multivariate testing.
We experimented with Optimizely but had a bad experience. It seemed as though their javascript was synchronous and blocking. We may have been able to customize that but did not dig deeper.
Another flaw in Google Content Experiments (which was mentioned in the article) is that it doesn't distribute traffic evenly. This is a big bummer with seasonal products/services where having the control start getting 80% of the traffic during a high conversion period makes the test useless. Overall, even though testing is easier in some ways, I definitely miss Google Website Optimizer.
