I think @Zoe has provided most of the answers already but here are some more perspectives from our end.
First, if you go with Agenda you do indeed decide to put your eggs in the basket of a small team of independent developers. That is a risk in that if Drew and I step in the same plane and it goes down, @zoe’s story would probably be relevant, which even in this horrible scenario would give you time to migrate though.
Also, I’d say it’s a myth to think it’s less of a risk with much larger companies, those too can ditch a product (Google Reader?!, Facebook ditching new products regularly, etc), get acquired, or close down without much warning. In fact, I’d say the fact that this is a small team without investors etc etc, makes we’re very much in control and very much dedicated and connected with Agenda, we couldn’t leave our users in the cold while still easily looking at ourselves in the mirror.
Finally, maybe looking at it from a different aspect, and just in case. Your data in Agenda isn’t locked in, we allow export in a variety of formats and even the raw data on disk is relatively accessible for anyone with a bit of technical knowledge.
Hope that helps!