I was kind of hoping Cresselia would be less broken than I expected, but no, it actually seems to be worse.
Ex. 1: I beat Teifu on the ladder with a CM Cress.
I've been on the ladder most of this evening, and I've noticed some interesting things about our new arrivals to UU:
1) The strange thing is that I've seen Cress used to greatest success as a boosting attacker, mostly because Cress can obviously more easily abuse Moonlight in UU. I suspect that when the ladder statistics come in for this month, we'll see a lot of Charge Beam or Calm Mind sets being used.
2) In my (although limited) experience, a +2 LO Raikou can 2HKO Cress with Shadow Ball, but it's fairly likely that it'll be T-waved in the process, rendering it completely useless for the rest of the match. I've noticed more Spiritomb running around, but Spiritomb is not quite as bulky as many believe and can even be setup fodder for special sweepers like Raikou. Cress also stops Moltres cold, which I find rather disturbing.
3) As a countermeasure, I've scattered some Toxic throughout my team, but dual-screen Cress can still setup screens even when poisoned, making frail sweepers (like Pory-Z, more on this later) much more deadly.
I think Cress might be the elusive Triple Suspect Threat.
4) As for Pory-Z, I don't see it as much of a problem--yet. The difference between Cress and Pory-Z is that the former requires little in the way of team support whereas the latter requires plenty. Interestingly, they seem to work well in concert; especially when Cress has Reflect/Light Screen/Lunar Dance/Filler and Pory-Z boosts via Nasty Plot. Other than this scenario, I haven't seen it as much of a threat as it is fairly easily revenge-killed by the tons of priority that have mysteriously cropped up in the last couple of hours. :p
All of this is simple observation: For all I know, I could be wrong about several things that I have outlined. However, Cresselia leaves me with a bad taste in my mouth as I've had to re-work my team not only to counter Cresselia, but also to have one. It's that centralizing.