I'm still trying to see the benefit of paying for a subscription to Garmin Pilot or Foreflight when Flightplan go is free and appears to work just as well. What am I missing by not spending my money?
For me, it's the integration and usability. I can create a flight plan on my iPad (and now on the web) and then, when I get into my plane, that plan is uploaded to my avionics seamlessly and in the same format I've been using on my iPad. GP also downloads and subsequently pushes routine updates to my avionics' databases, so I don't have to mess around with SD cards. The interface is intuitive (more so than FF, in my opinion) and does a fantastic job of keeping me aware of NOTAMs and TFRs (particularly important in the SFRA region). The avionics feed GP updates every second, including attitude, location, ADSB traffic, and engine parameters, all of which combine for useful tools in flight and an easy way to comprehensively review the flight (especially engine data) when back on the ground. All of that data is linked to logbook entries, including maps, in a native app on any device I care to use.
More edge-case, there's also an excellent simulator mode which allows me to link my iPad to my PC for flight simming. When I was planning my coast-to-coast, I was able to fly various routes in the simulator, and see GP reacting exactly as it would in flight. Great training device and when I finally flew the route in real life, it was like I'd already done it before.
Fltplan.com is a great resource. It's a little fiddly for my taste and lacks certain features (e.g., GP is showing a yellow "upcoming TFR" near where I'll be flying tomorrow, while Fltplan.com isn't showing anything there, even with the TFR layer turned on). I think it probably meets the basic needs of a lot of VFR pilots and is a completely reasonable option if it checks all the boxes for a particular aviator.
Given the sheer amount of money I spend on aviation, GP is an absolute drop in the bucket for the utility it gives me. For the price of gas on a decent cross-country, I get a year of a great product that makes flying easier and has been invaluable for maintenance. When my A&P shop wants to know what oil temps, CHTs, and so forth I'm seeing as we chase down an issue, I don't have to pull that from memory. I can show them graphs for all phases of flight, linking any parameter they care to examine. It's really something!