We built it by keeping things ultra-simple and released it with only the absolute necessary features. We even left out the ability to delete/edit your fuel records or the ability to recover your lost password. We launched the application three weeks ago and have spent nothing on marketing/advertising and currently have over 2,800 users. People really appreciate the simplicity and usefulness of the application.
Product Blog update: (37signals)
Yes, I am constantly annoyed by the added complexity of being able to recover my password and actually use a web app. Well played FuelFrog!