My Rex is at 6* purple gear lv70, skills ~6. I run him in a Clone team with Fives, Clone Sarge and Plo Koon. Let me tell you right away, the amount of turn meter manipulation you can get going with this team is crazy.
Rex, Plo Koon, Clone Sergeant, all have powerful turn meter manipulation. Fives can just spam slow on everybody. The fifth toon I alternate between old Ben and Royal Guard. Both work great in this team. With Rex in the lead, it is generally robust enough to withstand hits from high DPS toons. With Royal Guard, even more so.
The strengths of the team mitigate the weaknesses. Rather low attack power on the one hand, but relentless pressure and turn meter manipulation on the other. Rex definitely made Clones a viable alternative.