Values listed in brackets are for alternate (secondary) fire.
The M32 is a multi-shot grenade launcher with good damage and a good rate of fire. It has a load capacity of 6 grenades, and can thus deal a lot of damage in a short space of time. In comparison to the M79, it has a larger magazine size (the M79 is single-shot, requiring a reload for each shot), much better firing rate, and has a higher ammunition capacity. It does the same damage. However, it costs twice as much and weighs nearly twice as much (7 blocks compared to the M79's 4).
As with any explosives, target groups to take full advantage of the explosion radius.
Try to keep fully reloaded for times when you need all your shots to take out something big.