Is there any data that validates the notion that people will be leaving 5+ minute long messages for each other? I honestly can't remember the last time I left a voicemail for a friend/family, but that is purely anecdotal.
Anyhow, good luck!
Edit: But it breaks my heart to think that they will give it away for free to grow fast and then do all sorts of backflips in search of a business model. The business model is straightforward here. This product is useful. It has emotional value. Just charge good money for it and focus on making it bloody brilliant.
I'd love it if VoiceGem grew into a service that allows me to either listen to or read a message, depending on the context (in the car vs. in the office, high vs. low emotional value). Right now the best solution is to use Google Voice to transcribe voice mail and speech recognition to quickly type out an email, but it's not ideal.
Anyway, best of luck!
However, the use case you describe has come up several times now, so it's something we are aware of.
VoiceGem is for times you have a story to tell, but you don't need the other person on the line right that moment. You can send or reply to one whenever you want. I have received many great jokes or stories from folks in my VoiceGem inbox over the last couple days. That hasn't happened with voicemail before.
That said, isn't there already a very popular app that does this?