I have several of the cubes, as well as their life size prairie dogs made from the same material. Bullets that strike other than 90 degrees to the impact face CAN become lodged into them, but most pass right on through. Having a backstop is still a must, IMHO, as the pass throughs always seem to be redirected from the original line of fire.
They'll last a good long while, too. The lower the caliber, the higher the rounds count you'll be able to get through them before they chunk apart and/or get stuffed with lead. Fragmenting bullets, like the frangible varmint type, will usually fragment (just not as violently) and soft points will mushroom (but if they mushroom too fast they'll get stuck). I have recovered lots of bullets from the soft earth berm that I have as a backstop to support this. Bullets such as fail safes and partitions usually pass through, as do most FMJs, but the average soft point deer bullet usually will get stuck on a 90 degree strike in the center of the target.
Pistol bullets tend to hit and send the target flying/bouncing without penetrating, although on ocassion I have put a .357 sig and a 44 Magnum straight into the center of one. 9/40/45 do very little harm to it, but there's no telling which direction the bullet is going to get 'richocheted' after it hits the block(ask me how I know!!), so I don't recommend them for pistol work!
The Pdogs that I have leap pretty good with .17/.20/.22 centerfires, have to tether them to the ground to keep 'em in the shooting lane, usually! That's why I have 4, great fun to set up and knock down!
Tom