I signed up for the playtest a while back and downloaded the docs today. So far I have skimmed some of the play rules, looked at a couple of the pre-gen characters and spent some time looking through the bestiary.
Very brief first impressions: I like what I have seen so far a lot. "Old School" is a hard thing to put in a bottle but this feels a helluva lot closer to old school than 4e or even 3e ever did. If you doubt me just look at the bestiary for a while. I am pretty sure you could use the monsters as written with AD&D characters (or Labyrinth Lord) for that matter and have no problems. Of course the AC is ascending, but that is a cosmetic matter in my opinion. The monster abilities are very evocative and I got some good adventure ideas just reading through the bestiary. I cannot tell you how glad I was to see a "save or turn instantly and permanently into stone" after DMing 4e for a few years. I also like a lot about the character sheet. That cleric looks fun to play.
Even after less than an hour with the playtest I have an itch to both play and DM this thing.
I will definitely be spending some more time with this soon, perhaps a more thorough review will follow.