Having tested a few Circle missions on three characters, Ice/Nature Corr, SS/Regen Brute, and Merc/Thermal MM at +4x8 I will say I like most of the changes to the Circle. Obviously the mastermind struggled sometimes, but it wasn't too frequent and it was easy to recover. The variety in new enemy types makes them more interesting to fight.
I think the only thing I have an issue with the changes is on the rare occasion a group spawns with two ruin mages and two death mages and they decide to drop a double earthquake and tar patch and have enough CC to get through even a melee ATs mez protection. Maybe their spawns should be limited to 1 max per group? The exploding magi are definitely an issue for pets if/when they all explode one after the other, but enemies that explode have always been an issue for pets.
Overall, the Circle feel like a fair challenge now and more interesting to fight even if we do still need a lot of insight inspirations at times to fight off the -tohit.