I haven't gotten around with getting the colosseum monsters. The only one I got is Sazh and right now I'm about to do Requiem of a Goddess which lets me have a different version of Lightning. So how difficult are the dlc colosseum monsters? And are they worth getting?
Most of them are just challanges in post game... Most of them are fun but not a must-have. Snow's dlc gives you the only chance to to capture the unique "one-time" monster as often as you want. Pupu/kyokyo's ferallink however has a 999% sync-rate - the use of this fact is discussable because his dmg-output is pretty low. Gilgamesh/Valfodr are the hardest bosses in the game by far.
I like all of them - if u ask me give them a shot.
All the dlc fights are fun and some are even really challenging. They also give you tier 5 crystals, unique items and accessories, and allow you to tame unique monsters from the story mode. Also all the dlc monsters are unique and useful in some way provided you wish to use them.