Hi Mary!
I've recently transitioned to using only the Marketo Calendar for keeping track of my campaigns. It's been much easier to keep all the details in one place. Plus, with all the software products we offer, it's nice to have the advanced filtering to make sure we constantly have campaigns going out for each product within specific regions. So if my boss comes to me and says, "Have we sent out any press about PRODUCT X in the UK lately?" I can just go into the Calendar and filter by Location = UK, Type = Press, and Product = X. It's so simple compared to sorting through our project planning lists and Google calendars.
I, too, had problems adding non-Marketo items to the calendar. I'm still having a bit of a problem understanding the logic (ie: the part where you have to click on a campaign, then Program Focus on the event, then add the new item, which is then attributed to the original campaign you clicked on - it doesn't make sense to me).
That said, I'm enjoying the rest of the functionality and look forward to seeing where Marketo goes with it in the future.