Some gestures and navigation that Peek Calendar uses are intuitive; others are not. For example, entering the start and end times for an appointment requires you to tap and hold on the time field, before using sliders to set the proper time. It seems like it would be easier if you could tap on the screen and edit the time directly. Too avoid frustration with the app's many interface quirks, I strongly advise you to view the tutorial page linked to in the app's menu before you start entering events.
Finally, there's also a shake feature, where you can shake your iPhone and the app picks a random, kindhearted activity for you to complete. Some examples include: "Call someone you love," or "Take time to relax." It's a nice addition, I suppose, but I would prefer the app had a deeper feature set than these extras I really don't need.
Power users stay away
According to the App Store listing for Peek Calendar, the developer's caution that the app isn't designed for power users, and it urges power users not to use it as their main calendar app. That's certainly true, but the problem is, Peek Calendar is missing basic features that will require even casual users to use a second Calendar app to manage their schedule. That's hardly convenient.
Some of the more major omissions include the lack of search features, along with the inability to invite other attendees to a created event. A casual calendar without the ability to invite people to meetings makes some sense, but search features should be available in every calendar app.