Ichi is one of the best monsters in the game. Once you unlock it, start using them. At first, they may seem lame and weak, but they are little bags full of punches once at level 3. At higher levels, Ichi are more powerful with Zafreeti, because it heals the Ichi when they get damaged, making the ichis almost invulnrable untill the zafreeti dies.
It is usually better to use Ichi than Crabatrons as (1) Ichi is not affected by the well-feared Heavy Trap, (2) Ichi does more damage per housing space than Crabatron, (3) Ichi takes less time and Goo to make, and (4) Ichi is easier to train than Crabatron.
In the bunkers at a higher level, ichis are good if they are near Cannons or Lasers, because the ichis have a long distraction time. The ichi is also one of the most difficult monsters to bait, because they have high health.