What specifically makes it so cool in your opinion?
after a couple decades of breaking every cluster filesystem I've used at scale, Colossus was the first one that managed to handle my loads effortlessly. It required far less attention from SREs when abusive users sent it a lot of load. It also made using reed solomon codes easy.
The truly problematic Colossus clusters usually fell into one of two buckets: a) those with shrinking machine counts (but always increasing traffic) because the hardware was ancient and repairs/upgrades were not viable and b) those where most of the quota was held by a single project, with massive jobs. One cluster at IDI was mostly Image Search's and it wasn't fun at all.