In my testing, Soundbites worked as promised, though the audio quality left a lot to be desired--messages sounded more like lousy telephone connections than what I've come to expect from computer audio. I'm also not thrilled with what a chat window looks like after of a series of audio messages: it's just bubbles of audio files going back and forth, entirely inscrutable. Maybe some of that fancy speech-recognition technology could be brought to bear on these messages, so they were searchable (and even glanceable) without having to play them back one by one?
But there's still a lot to be said for asynchronous audio conversations. One of the reasons I text my friends and family instead of phoning them is that most of the time, I don't need to interrupt what they're doing right that second in order to get an immediate response--it's just not that important. With this new feature, especially on iOS, it'll be easy for us to hear each other's voices without demanding immediate attention with a phone call.
Group iMessaging also gets a big boost in Messages. Group messages get a new Details button (though the button looks like hyperlinked text, iOS 7 style, rather than a standard Mac interface element) that brings up a raft of options. You and your interlocutors can share your locations using the Find My Friends infrastructure, and the Details view will draw a map showing where every participant is located. You can also kick off phone calls, individual chats, or FaceTime sessions directly from the Details window, and add or remove participants. And just to help you keep track of your conversations, you can give each of your group chats a distinct name, like "Dinner Plans."
Perhaps most importantly, the new Messages lets you control group conversations that you might want to bail out of—or just not be interrupted by. You can select Do Not Disturb to no longer receive notifications from an ongoing group conversation, or click the Leave Conversation link to drop out completely.
Apple's Calendar app hasn't undergone as many changes in Yosemite, but there are a few worth noting. When you create a new event, it attempts to learn from previous events you've created and tries to autocomplete your event with likely dates and even attendees--if you often create a "Lunch with Jim" event on Tuesdays at 12:30, and you type "Lunch," Calendar will suggest Lunch with Jim, at 12:30, with Jim invited.
The Day view has been overhauled. It's still a two-pane view, but instead of the (fairly redundant) two daily schedule panes found in Mavericks, it's a single schedule pane and an inspector pane that shows you all the details of a selected calendar event. If you create a new event in this view, the inspector pane is where you enter in all the calendar information. It beats the cramped space in the floating inspector palette.
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