The ease of use, quick setup with any phone number/email address and no limits to how many people you can send it too! It also doesn't keep logs like some other services do so there's nothing stored about who sent what message or when they did it etc which I think is really useful from an investigation point of view if someone ever wants to know more details. A couple things are annoying - but not enough to stop using them completely- 1) Sometimes after sending one person in my group all others receive as well even though i've set this up only once per account2) Some phones (iphones especially at times?) seem unable to download updates without having internet connection available first3) When downloading via app - sometimes downloads fail4.) Notifications don't always work correctly.- However these issues aren't important enough to me personally to make me want to.