state of the art
Build infantry only on demand. Use 2 or 3 Barracks / Hands Of NOD to get them out quickly.
Infantry is a great force versus Recon Bikes: Infantry gets very little damage from Recon Bikes. Infantry cannot be squished by Recon Bikes. I.e. Infantry stops Recon Bikes like a wall. Infantry do good damage to Recon Bikes. Most NOD-Players rely on Recon-Bikes. But those Bikes suck versus Infantry (Minigun-Infantry, Rocket Soldier, Grenadier, Flame Thrower, Chem. Warrior). Don't use a Commando versus Bikes.
Don't build them to get squished by Your opponent's APC. I love to build an APC and drive through my opponent's base squishing soldiers worth a fortune. I have the APC empty. My opponent is in hurry to build more soldiers for killing engineers. But there is none in. He lose his money and I lose my driving license because of driving over people. Hey, they didn't use the sidewalk!
If You use infantry spread them with the "x"-key. Always. (I love stupids who position 5 soldiers on bridges or gaps. Keep them there, I'll visit You!) Attack tanks from all sides at once. So the tank can squish only single soldiers and get killed. I strongly recommend Rocket-Soldiers as anti-tank-force. Grenadiers have good power too, but they hit moving targets less accurately. Use the "g"-guard-mode for infantry. Let them select their target on their own. If You select their target they will follow the tank in a single group. The tank will turn around and squish all at once.
Infantry is very little used in multi play. I had a game (GDI-GDI) where I got engineered. He drove with 2 Engineers in an APC several times into my base. I couldn't prevent to lose my Weapons Factory and Construction Yard. But I had 2 Barracks! I built lots of Grenadiers and Rocket-Soldiers and won the game against all odds. I used the Grenadiers against his infantry and some buildings and the Rocket-Men against his Apaches, Tanks, Harvesters and buildings too. The key is to scatter the infantry. He relied on tanks and helis.
In another game (LAN: GDI-MacMark versus GDI and NOD) I won too. Why?
I lost all 2 Weapons Factories and the Construction Yard. But with 2 Barracks I won this game too. In both described games I lost 1 of the 2 Barracks too. But it doesn't matter. I strongly recommend (my favorite words!) having 2 barracks and building infantry only on demand. They come out very quickly and You don't waste money. In both games I finished them off with infantry taking out their bases.
infantry units (in alphabetical order)
A NOD only unit. Damages groups. The only infantry which is immune versus Tiberium gas. Sucks versus Apaches and armor. Good versus all kind of infantry and Recon Bikes.
Hits only single targets. Has greater fire-range than other infantry and kills a soldier with a single shot. Blows up buildings! Good versus buildings, infantry and whole bases. I blew up a NOD base with 6 Commandos. Look here and here (map: "Tiberium River"). Sucks versus vehicles and helis.
Hostile takeover of buildings. High tech buildings like Advanced Communications Center, Temple of NOD, Turret, Guard Tower, Advanced Guard Tower and SAM Site cannot be engineered. You can engineer a Chinook heli if empty.
A NOD only unit. Damages groups. Sucks versus Apaches and armor. Good versus infantry and Recon Bikes.
A GDI only unit. Damages groups. Sucks versus Apaches. Can destroy barriers. Good versus infantry, Recon Bikes and Tanks (Scatter a group of Grenadiers around the tank!).
Damages only single targets. Sucks versus Grenadiers, Flamethrower Infantry, Chem.-Warriors. Good versus Recon Bikes, Rocket Soldiers.
The only anti air infantry. Sucks versus other kinds of infantry. Can destroy barriers. Good versus helis, Recon Bikes and Tanks (Scatter a group of Rocket Soldiers around the tank!)