This new functionality is split testing on a whole new level and basically allows you to make changes on a keyword/bid level and see the exact before and after picture side-by-side.
A snippet from the Google release...
"AdWords Campaign Experiments (ACE) is a tool that allows you to accurately test and measure changes to your keywords, bids, ad groups and placements.
Today, the most common way to measure the impact of these types of changes is to do a simple before-and-after analysis. However, this type of analysis can often be complicated by events that occur during or between the before and after, including holidays, weekends or changes to end user behavior or advertiser behavior.
ACE allows you to test and measure changes in real-time by executing your experimental campaign alongside your original campaign. By performing this type of simultaneous split test, we can tell you precisely if your Search and Content campaign changes produce statistically significant results. "
Adwords Campaign Experiments (ACE) is currently only available for Beta testing to U.S. advertisers but I cannot wait until it is available over here in the UK.