Once I got my hands on the patch last nite I got right in. There was alot of large-scale pvp going on and it was pretty laggy in those areas but otherwise the world was stable with fairly low latency. Was even able to complete a supposedly bugged quest.
The last large outage they had was due to a major redesign of their back-end. I have to say that since then I haven't had a single problem due to lag or overpopulation and I am on one of the high-pop servers (Hellscream).
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Overall I think Blizzard is doing a good job, moving in the right direction, and is doing a decent job of dealing with the inherintly whiny MMO community. I've been in the game since Open Beta and I'll be the 1st person to say it should not have gone live when it did. But since then they've been steadily progressing toward a more stable, balanced, and complete game.
If this is your 1st MMO then I'm sure it seems appaling, but having played other MMO's I can tell you, to me, the progression has been above the norm.