I'm pretty much over open-world games. There have been so many since GTA3 and they are all pretty similar in many ways. Just Cause 3 is no exception, really - the story missions are the usual fare (here's a fetch mission, here's an escort mission, here's a rail-shooter mission, etc), and the general gameplay is what you'd expect. Speaking of story, I never played Just Cause 1 or 2, so I don't know the backstory, and the story cutscenes (while not exactly riddled with plot complexities) did seem to make several references to characters and events I didn't know anything about, so I presume a bunch of that was covered in earlier games. Another pet peeve I have with all these newfangled 3d games is loading time between cutscene... scenes... it really ruins the flow, and Just Cause 3 has plenty of it. A shame, since there are no loads during normal gameplay (outside of respawns).
Except it's almost the purest distillation of an open-world game with as many of the niceties as you could imagine added. You spawn with a full weapon loadout. You have incredible movement options pretty much right off the bat, with a great grappling hook, infinite parachute, and wingsuit combo. Using these makes almost all vehicular methods of conveyance irrelevant (helicopters can still be great for recon and long distance destruction).
Did I mention destruction? If you like explosions, this is the Micheal Bay movie of video games. Most of your game will probably be blowing huge explosive tanks to smithereens, which in turn blow other things up. It's quite fantastic. Unfortunately a lot of the bases and cities are quite cookie-cutter, so even the base destruction can get a bit repetitive - and since you have a LOT of bases and cities to 'liberate' (yes, you are blowing these things up to get them out of the hands of a military despot, very noble), you may find it a bit old fairly quickly.
On the plus side, the vistas you can see while wingsuit gliding way up in the mountains are beautiful. The engine can offer up some amazing sights.
I did enjoy my time with Just Cause 3, it did provide some truly entertaining moments, but the open-worldyness of it just makes it that much more repetitive than it could have otherwise been.