Some good comments here (http://apps.ycombinator.com/item?id=812764) about GWO versus more specific A/B testing tools.

The linked comment was by Patio11 (http://apps.ycombinator.com/user?id=patio11), who also has an A/B testing framework for Rails that's worth mentioning (http://www.bingocardcreator.com/abingo). I don't know how ABingo and Vanity compare.

Google web optimizer is a very cumbersome tool. Vanity is designed to work seamlessly in a Ruby/Rails environment. It even has a built in dashboard and reporting system! Also, its server-side, not client-side, which can really be useful in most cases.

I was very impressed with this when I saw it at RubyConf 09!

This is a rails gem, so you get all the benefits of integration with the web framework. As far as I know, GWO does A/B testing between hardcoded URLs, so the logic is then split between GWO (to redirect to the correct page) and your app (to serve the correct content for that page).

Ease of use, control, and not having to run the whole thing in javascript.