I saw it last night, and I really enjoyed it. I thought it was a much more intelligent film than any other zombie flick I've seen. It was coming in more from a global pandemic/eco parable kind of direction. It relied more on a constantly sustained sense of suspense, than gratuitous flesh-eating, which let's face it, we've seen before, many times. It was beautifully shot, showing off its collossal budget, and the acting was, whilst not in any way oscar-winning, completely fine - no weaknesses. I was honestly riveted from start to finish.
I think the luke-warm reception probably has more to do with the fact that it was Brad Pitt's project, cost an inordinate amount, and had such well-publicised problems (re-writes etc), and that it deviated a lot from the book, which is always guaranteed to raise the hackles of the purists. I think a lot of reviewers decided in advance that they were going to rubbish it, or get hung up on comparing it to other films/the book, etc, rather than just approaching it with an open mind, and taking it for what it is. It's not really a 'horror' film, but a very dark and interesting work nonetheless, not particularly following in the foosteps of any other zombie films. And yes, I loved 28 days later, but I don't see appreciation of the two as mutually incompatible. I don't know that I'm especially easily pleased, but I was totally entertained, and geniunely left the cinema feeling jumpy.