The flak cannon is the easiest weapon to get to grips with; and should be most novice's first choice weapon.
It is effective at close, and close to medium range.
Primary fire
has a wide scatter - so at close range most will hit. At a distance, even if you do hit a target; most of the shards will miss, and therefore damage will be minimal
Flak shards bounces off floor and walls, so can be used cleverly to shoot round corners. They are lethal in confined spaces.
Secondary fire
on contact with the ground or wall, it explodes
It causes considerable damage, and is more effective at a distance than flak primary
[edit]Tactical uses of the Flak Cannon in Onslaught
The flak barrels are the easiest way on foot to quickly damage stationary objects like power nodes
Flak shards have considerable momentum that will throw lighter Vehicles back and stop them from crushing you.
Flak is effective at damaging vehicles. Stepping to the side of an onrushing vehicle and using flak often works well.
The arc trajectory is useful to attack an area e.g. a node from behind ground cover; without exposing yourself to line of sight view of your target.
[edit]Tactical uses of the Flak Cannon in Deathmatch
My preferred use is to predict where my opponent will be in 3/4 of a second - and fire a flak barrel to hit that spot; then hit that same spot with a flak primary as soon as the barrel is launched. If your guess is right, both shots will hit your opposing player at the same time; which should deliver more damage than the opponent has health. This is particularly useful if your opponent hasn't seen you.
By alternating primary and secondary attacks in a duel; and correspondingly moving closer or further, your opponents will have difficulty in second guessing your moves; which is always a good thing.