A great experience with its flaws
Deus Ex introduces a large world filled with people and their different interests. The game doesn't judge anyone and you'll never get a Game Over message because of a decision you made. Naturally you won't be able to help everybody as some interests collide, yet without doing everything you'll still get over 10 hours of gameplay on your first run (you'll surely return for a second run).
The story of the game is long and interesting, and even just for that you'll keep playing till the very end, where you have to take a side and choose from three possible endings.
The game tries to give you as much freedom as possible within the boundaries of storytelling, and does a good job at that most of the time. You have 2-3 ways to approach each objective which you can choose from depending on your preference and skills you've acquired.
Deus Ex awards the player with skills point and augmentations - electronic upgrades to the player's body. This lets the player have full control over the development of the character and direct resources only to what he or she wants.
Not all augmentations can be applied to one player, and together with the multiple plot and approach choices you get a game worth playing more than once.
The biggest issue with Deus Ex is the AI. Even though it's not a game focused on stealth like Thief or Metal Gear Solid, you can't help but laugh sometimes at the stupidity of the enemy. An enemy soldier can stare at you for a few seconds before deciding to take action, and once he decides to engage you he just strafes a lot while shooting.
Also if an enemy soldier happens to hear you for a brief moment, he'll just mumble to himself something and won't bother inspecting the situation.
The lack of proper AI - in addition to no award for taking the stealth approach - makes the game a case of using the stupidity of the enemy for butchering an entire army one soldier after the other.
Deus Ex has a fair share of bugs, although they aren't critical and won't mess with your experience. Still after playing with the latest patch I can't help but feel the game rushed to release before going through final tweaks.
Although the game tells a good story and keeps you hooked in, most characters are shallow and the death of someone won't change the tone of the game. The often bad voice acting doesn't help make it better.
The Bottom Line
Deus Ex is a recommended experience to anyone - action, adventure, stealth and RPG players - but has a decent amount of problems.
Luckily those problems can be easily overlooked as they are mostly minor, and you still get the most important thing - solid and appealing gameplay.