As I understand it, Image has had a policy for a couple of years now where they don't solicit a new series until the first three issues are ready to go - in part to avoid reader frustration with stuff like Nonplayer where the first issue comes out and then it's months or years until the second.
I think that's a good idea, as in the early stages of a new series it can really kill reader interest when issues don't come out in a timely fashion. So it was nice to have the first three issues of this series come out on time, and get new Frank Quitely art on a monthly basis.
Personally though I don't worry too much about when comics are coming out these days - I read them when they're done, and until then, there's plenty of other stuff out there to occupy me.
I take Ben's point that this case is slightly different, as there was a specific promise that the entire series was completed before being solicited so there definitely wouldn't be any delays. So I can understand people being disappointed in a delay. But anyone paying close attention when the series launched would have seen that it couldn't be the case: Frank Quitely was still talking about finishing #4 and mentioned that he hadn't started on #5 when the first issue of volume 2 came out. So we knew a delay was likely.
I think with a book like this, at this stage in the story, it's definitely preferable to wait - and get art that's up to the same standard as the rest of the series, rather than rushing it or getting a different artist in. As was the case with other great books that have suffered with lateness - Watchmen or Ultimates or Planetary spring to mind - once they're complete, those scheduling problems fade away and are forgotten, but the work itself will continue to exist forever, so you want it to be as good as it can be, even if that comes at the expense of a quick release here and now.