I recently started using OnXmaps. Hands-down the best platform I've used to-date. Last week some friends and I spent a couple of days exploring the back-country near Bryce on ATV's and Dual-Sports and I used OnXMaps and my handheld Garmin GPS all day long. The things I like about OnXmaps are:
1) Easy to use (though most of them are)
2) Accuracy. Users can do real-time updates on Trail status or condition, which for us was a benefit since we had two days of heavy rain which in one case blocked-off access to the last three miles of a trail we wanted to ride. We were able to see that update right away.
3) You can see property lines and land owners info. I really like this feature as you can tell at a glance exactly what the status is of where you're riding. This feature came-in handy on my trip when a Karen-type accused my friends and I of riding donuts in her driveway ("I own all of this, you're trespassing, I can have you all arrested!") A quick check of my map showed that she did indeed own a piece of property on the road we were on, but she was not the only owner and it showed that we were on a County Road, not anywhere near her driveway. Once I showed her that, she chose not to stick around for my explanation of the difference between doing a donut and spining a Quad around within it's own length to turn another direction. Too bad as I was prepared to show that move as many times as it took until she understood the difference
4) Off-line map access. I'd download maps of the areas where I was planning-on going back where we were staying then could access the maps without needing an Internet connection.
5) Many different options for maps (Topo, elevation, etc.)
6) Trail ratings are pretty accurate (because they're updated regularly by other Users) plus it has a feature where it will show you trails near your present location.
Things I don't like about OnXMaps:
1) It's expensive (I think with an on-line discount it was $80 for a year). However I expect to get my money's worth out of it.
2) I don't like having to use my phone to navigate. If you could load it onto a hand-held GPS I'd be stoked.
Other than that it's still a new toy for me so there are lots of features I haven't played with yet, but so far I'm really digging it!