I realize that I am a tad bit late to the party here, as Agenda 3.0 came out about a week or so ago, but that doesn't change my enthusiasm about the update. I wrote a while back about the two contenders for the best iOS calendar app, but there is now no longer any competition. The one gripe I had about Agenda, and why at the time Calvetica won out for me, was that it used the default event entry screen — which I find to be rather cumbersome. Calvetica did not — it had it's own custom one.
The truth is, that even before the 3.0 update for Agenda came out I had switched to it just because, after using them both a bit more, that Agenda had the superior UI overall. And guess what, on top of all that, the latest update to Agenda brought? A custom event entry screen — which is far better than not only iOS's, but than Calvetica's as well.
Agenda is an absolutely excellent calendar app. I always loved the week view on the iPhone, and I now love how incredibly quickly I can add new events now with the 3.0 update as well.
Agenda is a universal app, and while I do like the iPhone version far more than the iPad one, the iPhone app alone is worth far more than it's $0.99 price tag. If you are looking for better iOS calendar app than Apple's, or want to see what you're missing, I highly recommend you check out Agenda.