Shakespeare's Sister has one of the best overviews of the Downing Street Memos out there. The worst quote, to my mind, comes from the Ricketts Memo: "[The United States has not] satisfactorily answered how that regime change is to be secured, and how there can be any certainty that the replacement regime will be better." The White House really had no clue what it was getting itself into, did it?
Meanwhile, there has been a bit of talk floating around that, like the CBS memos on Bush's TANG record, the Downing Street Memos might well be forged. The evidence? British reporter Michael Smith photocopied the original documents, and then destroyed the photocopies when he was done with them. Well, it sounds suspicious, but as Eugene Volokh points out, this seems precisely the sort of thing a reporter would have done if dealing with government documents. At any rate, if these documents exist, plenty of British officials have seen them, and not one has yet disputed their claims. At this point, it's safe to say they're real.