A mixed bag

First and foremost, this is an amazing app for free. Lots of great features: free geo-referenced sectionals, en route and approach charts. Compares favorably to Garmin Pilot for $150/year. Lacks terrain and obstacle data and warnings. Also no synthetic vision, attitude indicator or other AHRS features. Reportedly works with a number of external gps units but not with the Garmin GDL 39. Not sure if it supports ADS-B in features such as weather and traffic. Not surprised for a free app.
The most puzzling thing for me is that despite its name, there is no flight planning or editing in the app itself. You need to do all that on the website, and I find the website planning to be frequently confusing and un-intuitive. If you can stifle yourself long enough to learn the web planner, it does a great job and produces nice flight logs.
If you have room on your device, its a no brainer to install and use as a backup, if nothing else.
However, avoiding frustration and enjoying the benefits of a top-tier aviation app is worth the $150 for me. I have been using Garmin Pilot for two years. It started out with lots of rough edges and bugs, but has evolved into a first class all-purpose aviation app. Many of my friends use ForeFlight, which seems to match Garmin feature-for-feature.
My net recommendation is to flip a coin, or whatever, and choose either Garmin Pilot or ForeFlight, spend the $75-150 per year and enjoy the amazing features and utility that was unimaginable only a few years ago. If you absolutely cant afford that, or wont, then use the free app and resign yourself to limited features and utility.
Having one of these EFB products on board is absolutely mandatory.

Lindy2409 about FltPlan Go for iPhone, v4.1.1