I got my copy of Detective yesterday and after finishing it I was very disappointed.
I was happily reading the main story and then wham! it suddenly ended halfway through the book. I even knew the main story was going to be shorter thanks to there being a back-up story, but I didn't think it was going to be that short. The cover was also very misleading since the main story had nothing to do with a Batman/Penguin/control over Gotham rivalry. This new story arc definitely suffered for not having the usual amount of pages. The back-up story was actually better than I was expecting it to be, albeit not what I wanted or paid for.
If I hadn't read about it in this thread, I wouldn't have known about there being a back-up story in this issue beforehand. If I'd read this issue without knowing that, I think I'd be even more disappointed with it having not been prepared for a shorter main story. In fact, DC didn't even mention the story would be half it's size/have a back-up on the description page of the official site which I find to be a tad underhanded.
Anyway, if next months issue has a half-size story again then I won't be buying it and I'll be dropping Detective altogether.