I like how easy it is to use, especially when sending large attachments via e-mail! The "Save Draft" feature has been my favorite so far - just click save draft if there are any issues or questions about your message before hitting send (it also helps in case of loss/destruction).
It's not always intuitive at first but once figured out its great. Try using Flep for some time; even though this isn't an issue per se since we're primarily communicating through other means now, being able to have accessable drafts saves me from having to retype everything everytime i want something sent back & forth again between coworkers etc., which can be quite annoying sometimes as well :) As mentioned earlier by others too, make sure all parties involved understand what they need to do after receiving their copy / copies because often times people forget things unless reminded multiple times, esp those who haven't used flup much.