We have it implemented in some of our lead nurturing programs and here is what I know so far:
1 - It will not work very well out of the box - the history of a persons opens and clicks does not look back prior to having STO (send time optimization). This means the first time they encounter it, it will not have any history of when the person opens or clicks emails and will therefore send the email right then and there.
2 - PITFALL - There is no ability to setup a time the email would be sent within if send time optimization does not have history on the person. IMO this is an oversight on the creation of this feature.
WORKAROUND- We put in an step with an app right above the email send we were doing, the app is called "date calculator" which calculates a date and we made it place that date it into a temporary field we have - for only one reason - the date calculator step let's you wait until a specified time to run. The "WAIT" step in Eloqua only allows you to wait until a specific date and time so it is not an option for a nurture which is always running. Getting back to it, The date calculator is being used for the sole purpose of waiting until those days and that time range and then puts the person in the email send step which will have STO run and if STO does not know when to send the email it will send right away but be within a decent time frame.
3 - We have run tests and confirmed, it will send right away the first time and use some algorithm the second time if it has email open data but we could not decode the logic after doing various testing with new contacts where we were logging the opens and trying to decode how it works.
4 - Lastly, As most Eloqua awesome features, it is tainted with lack of documentation , clarity and reporting. Example, you cannot look at any contacts to see their STO settings or how Eloqua calculated the STO. You cannot find out how many contacts have STO data built. It seems like a black box to some degree.
If you have started using it, or anyone else, I would love to hear from you as well.