The inspiration for Optimizely came from my experience during the Obama campaign where I was the Director of Analytics. We used Google Website Optimizer extensively and even tried Omniture Test & Target.
The biggest pain point for us was setup time & implementation. We were all pretty technical guys but we only had so many hours in the day. Having to go back to the code base and remove add noscript blocks everywhere was a huge tax on our resources. Omniture has a similar pain called "mBox hell" caused by having to keep going back to your CMS or code base to make changes every time you want to run a new experiment.
We probably only had the opportunity to run 10% of the experiments we wanted to during the campaign. In the end it meant an incremental $60 million we were able to raise but I'm convinced that if we had Optimizely during the campaign that number would have been a lot bigger.
Google Website Optimizer is a great product but if you are short on time and don't want to muck with your code give Optimizely a try. Certainly let us know if you disagree!