Actually, I'm a little surprised how smooth it has been. I logged in around 2:40 am PDT and only had to wait a minute in the login queue - I was prepared to not get in at all (I think Moogle is one of the most popular EU servers). I also didn't get a single disconnection during the 6 hours and no major lags. Granted the lvl 18~22 areas are packed full off people and FATEs are pure madness. Also they could dynamically adjust the spawning rate of mobs on crowded areas, it's been a KSfest to do some quests (though having a full party doing the same quest against other parties tents to help a bit). I'm clad I'm on PC, since my buddy playing on PS3 says he really can't see anything in the FATEs etc.
I know how the ppl who want to create a character on a specific server feel, but you really just need to wait. Took me 8 hours refreshing the server list in Phase 4 before I was able to create my char on the Moogle... It's all the merrier in the end when you finally get to play :)
