I personally have never been interested in this series and they've never done anything to get me in. I know that's anathema to a lot of people, especially old-school PC gamers like myself, but I got super bored with the first one each of the 3 times I tried to play it. Just never got very far. None of the other entries ever felt like a good place to jump in, really, so I just sort of stayed out forever.
Part of my issue with these games is I've never really understood what they were about. Not really. All the stuff I'd see or read felt kind of vague and cryptic. Whatever they were doing for marketing, it wasn't very good, and that's a problem I see pretty frequently lately. I feel like too many AAA franchises are relying on names and either not making the interesting stuff the game is doing known, or the game in question just isn't actually doing anything interesting to begin with. Too much marketing is thrown at the names and not at anything substantive. I'm more interested in seeing something original, and AAA consistently delivers the least of that these days, it seems. I'm almost less inclined to look at a game if it's a franchise deal, to be honest.
Deus Ex series is a sci-fi / cyberpunk setting-based game.
Basically, it's usually about the Illuminati pulling off conspiracies. They are manipulating, setting-up, and controlling certain (horrible) events to gain more powerful so that they can control the world + those who inhabit it.
Usually, the character you play as is, at some point, to find out that they exist + must go up against them.
Gameplay-wise, Deus Ex series is a hybrid of a bunch of games. It's basically a FPS/RPG/Stealth/Action game hybrid.
You often can tackle one area numerous different ways, based on what skills, equipment, or whatever you have at your disposal.
For example - you could go guns blazing, through the front door, in one area. Or, you can sneak in through the vents + ducts. Or maybe, you can use a tool to hack the door open.
Once inside - maybe, you can stealth your way around, kill everyone, and not be seen. Or maybe, you can use non-lethal weapons and just knock everyone out + leave.
Or, maybe you can hack security systems, turn camera off, turn robots + guns on enemies, and let the area do the work for you.
Or, maybe you can mix + match a bunch of these styles.
Sometimes, you'll be given choices - and you have to make a decision w/ siding w/ a certain person, faction, or whatever.
Or you'll be given a choice - and a branching path can have 2 different results, at least; sometimes there's more.