I've only been using Calendars 5 for a few hours and I'm in love.
I'm sure I'll hate it eventually, but I usually do my hating faster than this. Calendars 5 is the best calendar app I've used since DateBk 5; it gives me hope I can survive Johnny ("The Designer") Ive's kneecapping of iOS 7 Calendar.app.
This is what we've needed. No #$@$@ wasted white space. This is a Calendar app that lets us actually, you know, see the friggin' appointments.
It synchronizes with any one or more of Google Calendars, Google Tasks, and the "native" iOS Calendar apps. I think the Google Calendar sync is using Google's native APIs; I didn't have to use Google's odd web page to setup CalDAV sync for multiple Google Calendars. i had only to enter my Google credentials and all my 15 or so Google personal, family, and subscribed calendars were immediately available.
But what about my Corporate Exchange Calendar? I figured that wouldn't work - but I was wrong! I enabled 'Native'/'Local' Calendars as well as Google (when I first connected it seemed I could do only one or the other, that's wrong) and I picked up my corporate calendar that way . (Calendar.app on my phone also has my Google Calendars, but native support is much nicer than CalDAV support so I disabled those.)
I don't use Google Tasks, but Emily has been reluctant to add the complexity of a separate task manager to her Calendaring. With Calendar 5 there's only one place to look.
List, Day, Week and Month view in vertical and horizontal layout are all effective on my iPhone 5. Did I mention Search in List view? Fast!
This is the Calendar app I've been waiting for. Not Cue, not Fantastical, not Agenda -- this one.
 I didn't try to edit that Exchange Calendar, that barely works at the best of times on Calendar.app.