I've waited for DeusEx:HR for a long time but half way through the game, I'm completely dissapointed. So here's my 0.02 dollars:
Tip 1: the all mighty Silencer. If you expect to be able to buy it from the store, get your ass on google and make sure you find out where it is in the game. Otherwise, you'll miss all of them. Like I almost did.
Tip 2: Don't spend weapon upgrades in anything but the Heavy Rifle and the 10 mm pistol. And only keep EMP grenades (and a few frags for the first boss). Everything else is useless.
I play the game on the hardest difficulty and the firefights are tough. Especially the bosses. But the AI is so bloody stupid that a big firefight is hard to find in this game. The experience you get from killing is a sad joke. Consider these tactics:
1. stealth your way in the base, bypass every enemy... no exp, except for the "ghost" achievement.
2. knock out every enemy, one by one, keep them alive, you get 50 exp per enemy, plus the "ghost" achiev.
3. silently kill every enemy by cutting them with a blade (30 exp) or shooting them in the head with a silenced weapon (20 exp), and you again get "ghost".
4. shoot every enemy with a rocket launcher then carefully move your ass 5 meters around the corner and wait for the guards to say the famous last words "all quiet now, nobody here", then repeat. You again get 20 exp per body and the freakin "ghost" achiev.
So, you'll be tempted to stealth your way through the game, open each stupid lock and computer, just to get those upgrade points. Unfortunately only a few of those are really useful (inventory, armor, aim and recoil and a bit of hacking), so you'll end up maxing out before you reach the half point of the game. And be bored out of existence by endless hacking scenes which give you absolutely nothing... I actually had to reinvent my style of play, from stealth to silenced pistol stealth. Only to find out just how bad the game really is. Once you no longer wait 10 minutes to clear a room silently, you realize just how small the levels are and how little there is to do or break, how linear the entire thing is and how boring and predictable the story is. Even with the awful graphics, I'd rather play the original Deus Ex. It was good for a reason.