The most unusual thing about Halting State is the way it's told: second person, from three perspectives. I did eventually adjust, but it took longer than I thought it would - it's a little hard to simply drop in and out of. It's also a slow start from a technobabble perspective - as expected from a novel about a technical crime, and one in which the protagonists are experts in their fields, it's a little heavy (overly heavy?) on the jargon to start with, sometimes needlessly. That said, past the first third, the pace accelerated rapidly and I found myself quickly absorbed.
The technothriller aspects of the novel - particularly later - are wonderful, and the way in which the initial crime fits into the larger picture is perfect: as always, Stross' worldbuilding (even in the near future) is both inventive and compelling. But what about characterisation? Despite enjoying Halting State, that's where my second gripe comes in: while both Elaine and Jack's plotlines had clear structures, Sue Smith's perspectives seemed simply there to keep an eye on the police side of things, and tie the others together. In other words, her role seemed far smaller than expected, and I was occasionally left wondering why such a large chunk of the novel was devoted to her viewpoints - while I liked her as a character, it would be nice to have seen her have a more active effect on the plot. That said, all three characters worked well in the setting, and I found Elaine particularly engaging: a businesswomen essentially hung out to dry on a troublesome case by her partners, she's nonetheless determined to make sense of the situation, and definitely manages to take charge. There's also a nice revelation with Jack's character that I didn't see coming, and succeeds - as all good twists should - in really turning things around.
The pacing, meanwhile - post the first third - is well tuned: we're left just enough time to accustom ourselves to a new mystery before the whole issue deepens. And it's just just investigation, don't worry - there's some... inventive (no spoilers, I promise!) action as well. I especially liked how parts of it are set up from the beginning in the worldbuilding: it's not gratuitous, it fits, and more, it clicks (or at least gives you the "I should have seen that coming, but I really couldn't!" feeling, which is the best feeling).
Overall though, Halting State is well worth reading. If you want a fun, well-developed mystery - with some unusual and interesting protagonists thrown in - I'd advise checking it out, with the caveats that it has a slow start, an odd point of view structure, and Sue's plotline meanders a little. While I'd never recommend it over the Laundry series (so far my favourite of Stross' - though I'm looking forward to the Mo book), it'll definitely pass the time enjoyably.