I have a paid subscription to AllTrails Pro, and I also use (and have donated to) Caltopo.com. I've also owned a few of the older Trails Illustrated TOPO! DVDs as well as Delorme's Topo North America product.
In places where Trails Illustrated has coverage, the map data (In AllTrails Pro) is really really good. It can be significantly better than USGS Topo or USFS data when you are looking for hiking trails, trail heads, camp sites, etc. This gives it a big leg up on competing products.
Some things I do not like about it:
When printing maps, there's no way that I know of to specify the map scale. You get a pre-set set of scales to choose from, and they are often non-standard scales. This doesn't mean much to most people. However, I know how to use and plot UTM coordinates, and this requires using a UTM plotter built to match the scale of your paper map. UTM plotters are built to the most popular map scales: 1:24,000, 1:25,000, 1:50,000, 1:62,250, etc. So it's frustrating to me that with AllTrails, I have no control over my map scale choices. The free online tool Caltopo.com offers "sane" map scale options, and the ability to choose an arbitrary scale defined by the user.
Second, I don't like the route drawing capabilities as much as what's offered by Caltopo.com. I find it much easier to draw, edit, resample, split, and join routes in Caltopo than I can in AllTrails.
The elevation profile statistics for a chosen route are user friendly in AllTrails Pro, but Caltopo has added some very sophisticated route statistics now that are quite interesting.
I haven't played with the AllTrails "smartphone App" feature, so I have no opinion about it.
There's nothing unique about AllTrails's ability to export .GPX files for use on GPS receivers.
The "social networking" aspect of AllTrails, offering the ability to share routes with others is interesting, but not unique. TrimbleOutdoors.com has a similar feature which enjoys widespread use.
The only compelling feature of AllTrails that keeps me paying is the Trails Illustrated map Data. But that's a mighty compelling feature! For everything else, I prefer Caltopo.