Yeah, I was in the middle of importing my Google calendars and while attempting to create a new calendar I ended up with a 999 error. I got that probably two hours and it hasn't cleared up yet, but I'm trying to be patient (I had a meeting for about an hour of that time anyway).
Overall though, I'm liking what they've come up with so far. I love that there's an integrated to-do list, though I'm hoping more functionality is coming for that (it seems to place them in a folder that's tied to the name of the default calendar, which is kind of weird to me, and I can't find a way to add new folders or due dates, so I'm hoping those are things that are to come).
I just read the Yodel Anecdotal post, and am glad to see "And soon your Yahoo! Calendar will even auto-synch with Outlook and your iPhone."
I'm just hoping soon is before my MobileMe trial expires in a couple months so I can confirm whether I'll want to pay up for it or just stick with Yahoo.
Now I'm just hoping this error clears up soon so I can add the rest of my calendars!