For an end user using our product right now the loop is basically:
* See the "new" badge or hear about an experiment that is available
* Visit Test Pilot and enable the experiment
* Maybe some kind of feedback or participation
* Eventually a concluding document and the add-on goes away (or it may go to AMO)
Participants get a pat on the back, a thanks for helping out, and that's all they hear. What if, for successful experiments, we left an artifact behind on profiles? For example, something in about:config. Then in six months when it launches in Firefox we could use SHIELD to send a notice to that browser that says something like "Remember Tab Center? Thanks to your feedback it's now usable just by going to about:preferences and enabling the checkbox." (If they are running nightly, they get that notice right away, otherwise the notice rolls out with the trains.)
The wording isn't perfect but if we leave simple prefs in about:config we have a way of reconnecting with folks and letting them know what happened with the project. Rather than just "thanks, the experiment is ending now" it would be "thanks, your participation made this tangible change that you can see right now."