I really enjoyed the Deus Ex games. Is Stalker similar?
No they are different. Deus Ex is linear following a story in specific zone with some choice and consequences along the way. You also earn experience points, augment and upgrade your augmentation in Deux Ex.
STALKER is open world made of many large zones that are connected through many paths that you can access any time. This is Chernoble after nuclear meltdown with strange things happening and you have to discover why you ended up there and why you were supposed to kill some STALKER (scavengers in these nuclear contaminated zones).
There is a main quest and many optional side quests that you can tackle in any way.
You don't have experience points to upgrade you skills. You have strange 'artifacts' scattered in the different zones that upgrade you health, armour, protection against radiation, fire .. etc.
You earn and find money to buy new weapons and upgrade.
There is huge emphasis on exploration to find new ways to achieve objective and find powerful artifacts, armour and weapons.
There are factions that you can align with in the zones (and to piss off!). The world and npc are simulated each action has a reaction on npcs and factions, which leads to some quests being removed and so on (choices and consequences).
The atmosphere is that of true horror and grim of the unknown (and I mean true horror).
STALKER: SOC is very ambitious and had many patches to fix. There are still some bugs, but is very stable and the engine (x-ray) is a PC engine that is very scalable and brilliant. This is a true gem.