Smasandian
Smell the Ashes
There is two maps (similar to how Shadows of Mordor did it) and they feel quite sizable.
You have a base of operations on a oil platform and you chopper into the two maps. It's a sandbox but there doesn't seem to be a lot to do throughout the map without involving missions. It's not Watch Dogs or GTA in that regard. There is main missions that take place in certain locations and side ops that does as well. You can free roam but I'm unsure why you would do that. There is materials, diamonds and other things you can grab that help you build up Mother Base (your base of operations).
In the end, your basically completing story missions and side ops to help build your base up. This works into the customization. I'm pretty sure you can't customize your guns like you can in Far Cry and etc but you can upgrade mostly everything you use in the game to different levels. This is based on your Mother Base research level, support level, cash and materials you get. Each level gives you different things like better accuracy, more damage or crazy things like your fulton system turning into a teleport.
I would compare the game to Far Cry but with much more stealth approach. The first mission you take on (which is the preview) is eliminating a captain in a village. I dropped in during the night (there is a day/night cycle), found high ground, zoomed and marked all the guys I saw. Slowly crept in, took out a few guys and fultoned them up to get rid of the bodies and use them on base (but the guards can still see the guns that were dropped) and saw the captain inside. I slowly crept in and took him out with a tranq gun and picked him up to get him out and use him on the base. During that time inside, a few guys saw something and I hid behind a bed and they left. While that was happening, a sandstorm started to happen which allowed me to quickly walk through the compound with the guy on my back and nobody could see me. I called the support chopper in, found a landing zone and put him on the chopper so I could use him on my base. I was going to fulton him out but there was 30% survival rate so I thought it would be best to get him on the helicopter. The sandstorm was cool but I know it was a scripted event because I reloaded once and it happened at the same time.
It's hard to say if its like Splinter Cell. The prologue showcased how a mission might be but Splinter Cell is a very scripted story experience. Don't get me wrong, I love Splinter Cell but it's not as open as this game. MGS has a ton of little tricks you can do (not sure if they are meaningful or not) while Splinter Cell has its stealth gameplay nailed down tight (which is a good thing). Personally, if you thought the demo was damn cool then I'm pretty sure you will like the game. There hasn't been an open world game with this type of stealth mechanics. Far Cry had it but it was pretty standard stuff.