For me, the biggest drawback to using my phone as a GPS is that it sucks the battery life rather quickly. Gaia might suck up something like 75% battery life on a 4 hour hike. iPhones aren't really known for their battery life though. It's a non-issue when I'm wheeling because I just keep the phone plugged in.
Still not a bad deal for $20 though. And when I'm hiking, I only have to carry two electronic devices rather than three.
A few screenshots from Gaia just to give an idea. Red is an actual track I recorded. These maps can all be zoomed in more. There are other maps available too (topo, hiking, etc).
US Forest Service:
Worth noting that my husband prefers Motion X for hiking. It seems to be a bit more accurate on the distance calculations. I don't find it to have as many useful features as Gaia for wheeling.