The API docs need some love still (http://developer.close.io/) but the API itself is very powerful since we use the same API for everything in Close.io itself.
Thanks for the link! Do you think you would ever add a physical address to 'Opportunity'?
We allow comments on Opportunities. Eventually we'd allow custom schemas here but for now mailing/physical addresses get stuck under "Leads" (aka Accounts) (which Opportunities belong to)