The cob cannon is a plant that can launch corn bombs onto anywhere on the screen (one corn bomb is equivalent to one cherry bomb). However, the cob cannon can easily be destroyed, due to its large hitbox and costs a lot of sun (700) with the kernel pults.(Plus you can't pumpkin it.) While the cherry bomb is only 150 sun and is invincible (due to being a instant kill, it can only be stolen by bungees). Cob cannon's only advantages were not being able to be stolen (Yes, the cob cannons size makes it un-stealable) and having a shorter time to recharge a corn bomb (around 31 seconds while the cherry bomb takes 50 seconds) at the cost of costing 550 more sun. So, is the cob cannon worth it?
There are a few advantages you are forgetting:
- The Cob Cannon does not need to be planted where to attack. So it can be launched on top of a defensive Tallnut(also allows to be fired on ice, or without a lily pad/ flower pot).
- It recharges much faster then a cherry bomb giving you a much more reliable panic button.
- It is reusable, so depending on how long the round last they can eventually (after 5 cherries) be more cost efficient (especially if you take in consideration that 2 kernel's can be a cheap early defense line).
- You can have more then one, so with massive waves you can clear them out much more easily then with a cherry bomb.
So what we can take from this is that they are much better, if you got the time to use them. So personally i would use them on long games like Surival Endless, but other then that they are barely worth the effort.
In a regular level, no. There's no regular level long enough for you to make back your investment (two spaces, 700 sun, plus initial waiting time before you're able to fire it) compared to using a Cherry Bomb, Jalapeño, Boom-shroom, or Squash instead in that slot.
However, in Endless mode, these are pretty much a requirement to get much past wave 10. When the zombies get numerous enough a lot of plants are no longer worth it. There are enough and tanky enough zombies on the screen that the only way to stay alive is through AoE damage; single-target won't work. Since some zombies will outrange puff-shrooms and spikes (especially gargantuars can be problematic), melon-pults are slow, and there aren't enough one-use kill plants to have a continuous stream of them, 2-6 cob cannons are typically used as a supplement.
Some strategies will use them only as emergency buttons, while others rely on them in normal operations. When you want to go for 1,000 waves, it's good to have some insurance.