Mechs are slower, more expensive infantry that can hold its own against most
direct units like recons, tanks, and anti-air, while doing significant damage to
indirect units like artillery and rockets. Mech units can capture cities, bases,
airports, seaports, and HQs, traverse most terrain types, and have superior
firepower compared to infantry.
Mechs should be used to attack mechs, recons, tanks, anti-air, artillery, and
rockets. The bazooka can be used three times against those units and then you'll
need to bring an APC to reload the bazooka, or have the mech reload at a city.
Mechs can also attack infantry, but it's better to use infantry against other
infantry due to the lower deployment cost of infantry. Mechs can run into trouble
trying to get into range of an artillery or a rocket to attack it, but other units
can be cannon fodder as your mech makes it over, or you can make use of a
transport like an APC or transport copter.
In Fog of War situations, put a mech on a mountain and they will be as effective
as a recon. Sami's mechs in particular are lethal, particularly when Sami has a CO
power going, because of Sami's +1 movement on a CO power.