Stronger, slower, and more expensive than Infantry, Mechanized Infantry (more commonly referred to as Mechs) are still relatively inexpensive and versatile units.
Like Infantry, Mechs are also able to Capture Properties on the map. When given a choice, it is generally preferable to have Infantry commit to capturing over Mechs, as Mechs are slower than Infantry, but much more powerful offensively.
Offensively, Mechs are comparable to Tanks in terms of power, allowing them to do moderate amounts of damage to lightly armored ground units if allowed to get the first strike. Mechs will use their Machine Guns against B-Copters, however, so it is not advisable to attack B-Copters with Mechs. Against foot soldiers, Mechs are stronger than Infantry both offensively and defensively.
While stronger than Infantry defensively, Mechs still have relatively low defense and rely on heavy terrain to survive. Combined with their much higher cost relative to Infantry, Mechs make poor blocking units. They do make strong supporting units, however, deterring opposing light units from attacking your more valuable units without suffering a retaliatory attack from a Mech.
Due to their low movement, Mechs are out-ranged by almost any other direct or indirect-combat unit capable of hitting them.
Infantry are generally the most efficient unit for countering Mechs, being much cheaper while still doing reasonably high damage to them.
B-Copters are also strong at attacking Mechs, as they suffer minimal damage from the Mech's counterattack.
Tanks are situationally useful against Mechs, but they will take moderate damage from the counterattack, especially if the Mech in question is on high-defense terrain.
Anti-Airs are just as strong against Mechs as they are against Infantry, but will suffer heavy damage from Mechs if they are allowed to get the first strike.
Mechs do low amounts of damage to any of the more heavily armored units, but using Medium Tanks solely to counter Mechs is highly cost inefficient.
If an opponent is building large quantities of Mechs (often dubbed Mech Flooding), the best way to counter is to take advantage of the fact that they will be left with a relatively immobile army with poor map control. Groups of Infantry backed up by Artillery are effective at dealing with Mech Floods, being a more mobile combination of units with similar cost. Care must be taken not to leave any Artillery exposed to Mechs, as large groups of Mechs can easily overrun unprotected Infantry.