Well. Thanks to a mix of slow internet (2 Mb/s max) and a super buggy beta launcher, I am yet to actually log in to the Mists of Pandaria beta.
The first night I started downloading and went to bed. When I woke up it was at about 3% - it crashed shortly after I went to bed.
The next five or so times I went to reset it, it would get stuck at Initializing or Synching files or something like that. I had to end a ghost process, delete the battle.net folder in ProgramData, and restart my computer in different combinations before it finally got going again.
Shortly after, it crashed again and I had to go through the same thing as above.
Today, it was at roughly 55%, so I decided to try logging in, seeing as you can play past about 10% and it downloads files in the background. After sitting on the 'retrieving characters' screen for about the ten minutes, the character creation screen finally popped up...and as soon as I clicked 'create new character' I got a critical error.
After doing the stuff above for about 10 more minutes, it finally started again. But this time, it started at 55% while in the setup stage (aka not able to play while background downloading), with optimal (playable) at about 85%.
tl;dr, extremely buggy launcher. 2/10 would not bang.
I should be able to - knock on wood - play and give some initial thoughts by tomorrow.