I have used it a bit.
My general feeling is that the technical knowledge required to make your own maps might be a bit overwhelming for students if you were to just give them one assignment. However, if you had a larger unit in which you could build up the skills over time then I think they could adapt to it quickly.
edit: after looking through the GeoInquiries closer, I think the Darwin one is the one I used! I didn’t know there were lesson plans for it though. I will definitely check out some of the other Env Science ones.
What I have used in my class this year (and the Social Studies teacher has used as well) is their Story Maps I used one that another teacher built about Darwin’s voyage on the HMS Beagle. The map used by our Social Studies teacher is one built by the ESRI team about Martin Luther King Jr.
There is a searchable gallery and some of the content is absolutely awesome.
It might be something worth exploring as well–I haven’t tried to make my own story map yet.