It's possible that the tricky programming of Calendars is in the background, running the sync process. But the trick Google Calendar users will most appreciate is in the interface, which mimics its Web brother enough to be thoroughly familiar (more so on the iPad, naturally), while staying true to what we've come to expect from good mobile touchscreen design.
Like Google Calendar, the app enables you to choose from list, day, week, and month views; you can also choose what view you'll see when you first open Calendars, or default to the view used when you ended your last session. Calendars does not allow you to use the two-week view, which may only inconvenience some users; it is my personal default Google Calendar setting, and I hope this view become available in a future version.
When you first set up Calendars, the app imports information from your Google Calendar, including your personal (color-coded) calendars such as Work, Home, Personal, and so forth. It also imports your already-entered schedule and tasks. The app enables you to set up defaults for many things: pre-set e-mail, SMS, and push alerts; time-precision (when you tap the calendar, do you want it to estimate the time you want within five minutes, 30 minutes, or an hour?); whether you want your week to start on Sunday or Monday, and more. However, it doesn't provide access to many capabilities available in Google Calendar, such as the ability to add new calendars, or share and subscribe to calendars. You also are unable to create custom notifications for personal calendars, or choose to show or hide personal calendars.
However, Readdle doesn't tout Calendars as a standalone app, which is why it's called "Calendars -- Google Calendar client." While the app has some powerful capabilities and an excellent interface, Google's server still does most of the heavy lifting.
In addition, Calendars is not flawless; several times during my testing it exhibited syncing problems, and returned some unhelpful error messages. Also, it does not sync automatically--you must choose "Sync" from a menu before events and other information are transferred between calendars. It seems that a Calendar client should automatically sync when it is connected to the Web via 3G or Wi-Fi, at least at the end of each session, or even upon entry of new events and tasks.
On the iPhone, Calendars is naturally more cramped, but the design is similar enough to the iPad version (and Google Calendar) to make it simple to use. Running on the iPhone or iPod touch, the app provides the same options as those available on the iPad. It would be nice if there was a way to sync defaults (such as reminder times and types, display options) between devices and Google Calendar, but as of yet this is not possible. Perhaps there's not enough demand for such a service, as screen size plays an important role in default choices.
But overall, Calendars is a solid and dependable app, and these two traits are perhaps the most important you can ask for in a calendar. It can still use some improvement, and Readdle seems committed to regular, non-trivial updates: the recent 2.1 update, for example, enabled the editing of events in the iPhone calendar, and now allows users to drag and drop recurring events. Calendars has become one of the half-dozen or so apps I depend on to get my work done, and other Google Calendar users may well find it just as useful.