Project X is an excellent anti-tower monster. Project X at first may seem like a weak monster, but that's only you don't have 10 Million Putty and Acid Spores. Add 10 Million Putty to a group of 25 Project X and watch them tear down the defenses! But be careful, Only bases with the defensive towers close together and not many fortifications will be susceptible to this method. To help maximise the time the Project Xs are under putty rage, only fling in the putty when they are just outside the yard's tower range - and if possible, target the Aerial Defense Towers as this will make attacking in subsequent waves a lot easier.
An alternative method, which is also considerably cheaper, is to send in a group (this does not have to be very large) of Ichis/Crabatrons supported by a Zafreeti so the tower targets the Ichis/Crabatrons first. You then send in a couple of Project X (preferably from another side if you are attacking a splash tower) and watch as they destroy the tower while it shoots in vain at the group of Ichis/Crabatrons. This is very useful for clearing NECs (Never Ending Chains) as found in Donut yard designs, and clears the way for another wave to destroy the harvesters in the NEC. However, you want to be very careful of Booby Traps, as these could decimate your Project Xs if you are not careful.
The PX is an average defending monster. One cool thing about him in the bunker is that he does heaps of damage, but his health does not hold up long enough for him to be able to strike multiple shots. His acid spores make him do more damage, but when destroyed from a distance using Rocket DAVE(s) this ability cannot be used to harm the DAVE(s) unless you can draw them near the Project X.