I've just finished a 5 hour stretch. I was originally in the "This game blows" camp, but with some friends, I had a really good game!
I pre-ordered, and was really underwhelmed by it.. but I'm quite impressed at the work since then. It's starting to feel like they've applied a bit of polish.
100 man servers are in testing now, there's 15 or so up, all others are 50, XP is gained, some people are supposedly trialing skill trees... looks like the Steam uproar was just what it needed!
I think I'll definitely be playing in the future.
They've fixed the Zombie Sparsity issue too!