Overall, this is a pretty good offering. For me, it won’t replace the default app for general use, but it will stay loaded for some special use cases. Everything below is based on use cases for an iPhone 5 in San Diego, California.

Pros

The location data is unquestionably the best around. Google’s invested a lot in its mapping, and it shows.
The vector tiles are a huge improvement over the previous (iOS 5) maps app.
The overall design is clean and (mostly) consistant with Google’s other recent offerings like Gmail, Google+, etc. Some of the language differs from Google’s other offerings (two finger swipe to open the menu, for example).
Streetview. Enough said. Regardless of your view of the mapping data and how it works in your location, no one else is even close to being able to offer this.
Voice navigation is a welcome addition. The voice is clean, and easily understood. Will likely sound more natural than Siri to most users, at least, it does to me.
Traffic data is superior to Apple’s maps thus far, in limited use.

Cons

Biggest issue for me: no ability to integrate with Siri. I don’t use Siri for much, but for navigation while driving, it’s a godsend of a feature. Not being able to voice navigate handsfree while driving makes this a non-starter for me when compared to Apple’s app.
The performance on my iPhone 5 is unquestionably slower than the default maps app over the same WIFI network. Over LTE, I have the same experience. I suspect this is simply an issue of Google having more data to push to the user.
The design language is inconsistant. Sometimes it doesn’t use the proper language for iOS (such as when trying to share a destination, which should normally bring up a list of ways to share in a default window), but sometimes it simply doesn’t work the same way Google’s other offerings do. The menu is on the right instead of the left. The button to open menu options is at the bottom right instead of the top left. Etc.
Obviously, it cannot be used as the default app. This isn’t really a ding against Google’s implementation so much as it’s a problem with the inability to specify different default apps in iOS.
The sharing options are extremely limited. Locations can only be shared to message, mail or clipboard. Apple maps aren’t much better, but do also include twitter and facebook as default options.
Yelp’s location reviews are vastly superior to Zaggat in my area (US, San Diego). Your mileage may vary.
Shake to send feedback is on by default. I’m not sure who thinks that’s a good idea in an app that will be open while attached to a car while driving. You should turn this off no matter what if you use Google Maps.
No night mode. Why both Apple and Google are both missing this is beyond comprehension. It’s like no one at either of these companies has ever driven with their phone on at night. Bright white screen + car dashboard + driver field of view = crazy.