Well, just sitting here playing with FltPlan.com app at my desk and here's a few observations:
1. It's free - great! If nothing else get it for that. It's worth the price for sure.
Downloading maps failed several times for me. I do not know if this is always a manual process or if it reminds you when data is about to expire... maybe someone who uses it can explain this aspect.
Overall, it's pretty clunky. It shows a chart, which really is 90% of what you want/need. If you go complete a fltplan.com flight plan, then you can go back into the map, select that plan and it will show the highlighted path. However there's no way that I see to build a flight log from the map. The map is not clickable at all to add waypoints, new legs, etc. I do not see how you can edit a planned route at all if you have a re-route or just change your mind. Whereas in AviationMaps, if I click and hold any spot on the map I can add a waypoint, go direct, insert or append it to my current flight log.
Image quality is pretty poor. It's noticeably fuzzy/grainy at all zoom levels.
Here's a shot of my local area in FltPlan:
Here's a shot in AviationMaps
It gets a lot worse when you zoom out in FltPlan's app.
There's no leg info that I can see, ETE/ETA to next fix, etc.
In AviationMaps you can toggle METAR indicators as a layer on the map. TFR's are overlaid (like this one that just popped up on my route to Missoula). You can also see the extended runway centerline indicators at my destination showing up.
If you want airport info in FltPlan's app you exit the map and go pull it up in the AFD PDF manually. In AviationMaps, you touch the airport and basic info pops up in a dialog and if you want extended AFD info, airport diagrams, or approach plates, they are right there. The quick click airport info box in AviationMaps, with links to pull up extended info, full weather, plates, etc.
I'll try to test out the FltPlan.com in flight, but just from the basics I've seen so far I'd gladly spend the $4.95/month for AviationMaps. If you have nothing else, by all means get FltPlan.com's app.
Edit: with more playing I see more layer options to add TFR's and weather to FltPlan.com app, so that's great...also it does seem you can edit a route, you have to enable editing from a menu, choose the fix, choose what you want to do (edit/delete) and then you can click the map and it seems to search for fixes... I'm having troubles getting it to work, slow and lots of clicking, but it does seem that the functionality is there.