"Skulduggery Pleasant: The End of the World" is a 15,000 word novelette/novella written especially for the 2012 World Book Day event. As such, it's not a full novel, or a part of the overall series arc, nor does it pretend to be either. It's more comparable to the extra short stories that have appeared in the mass market paperback editions of the first five novels in the series.
There are several reasons why short stories set in the Skulduggery Pleasant universe are not, for me anyway, quite as satisfying as the novels - I like the jokes and the magic and all that, but what makes me a fan of the series is the overall series arc and the character development, and the short stories are naturally limited as far as both of those things go.
So if you approach "The End of the World" with high expectations, expecting it to follow on from the events of "Death Bringer", without keeping in mind the natural limitations of the shorter format, you might well end up being a bit disappointed. On the other hand, if you approach it as just an extra story set in the familiar universe, it's much more likely to deliver a satisfying experience. It's a *fun* story. Not deep, nothing to do with the series arc, and while it obviously takes place after "Death Bringer", it's very clearly designed to be read by anyone, including people with no prior familiarity with the series (being part of the World Book Day selection, it's quite likely that it will be picked up by people who haven't read the series).
It's exactly as an introduction to the series that I think it works best. We get the story from the point of view of 15-year-old Ryan, a brand new character, who knows nothing about magic or Valkyrie or Skulduggery, so the readers get introduced to those concepts and characters via his eyes. There is the trademark series humour, there is fighting, there is magic - if the new reader likes this book, he or she will likely enjoy the series. (And vice versa; although the series itself is much deeper, much more multi-layered, much more fascinating and with characters who are really much more rounded and interesting than a short story can show, if the humour and general feeling isn't appealing, then the series probably wouldn't work well either.)
All in all, I was happy with it and enjoyed "The End of the World" very much. Is it as good as the novels? No. Is it my favourite of the extra short stories? I'm not sure it's even that. But it was a fun little story set in a universe I love, and I appreciated the chance of getting a glimpse of Skulduggery and Valkyrie going about their daily business, and having a an extra little adventure to tide me over until the release of the seventh book.