Simply one of the best spigot forks for 1.8 out there.
I like how there are different types of knockback profiles. People that do not know much about knockback can easily configure one of the less detailed profiles while people who do know a lot about knockback have the option to customise it exactly they want it. With the "detailed" knockback profile I was able to create knockback which is close to MMC's sumo and boxing knockback.
I have heard from others that the performance is great. My server did not have that many players so I cannot really comment on this.
Finally, I'd like to mention that not every release was always stable. I had been using an older version (2.3.6) for a while because of this fact, but the developers of this product are very active and push fixes as fast as possible.
Carbon 4 seems to be working fine and I recommend anyone who is looking for a good pvp core to buy this product.