The first of a trilogy, The Accidental War spends a lot of time setting up the circumstances for the followup novels. As such, some may be put off by the occasional aside intended to provide necessary background. But I found the asides relevant and brief enough to not detract from the pace.
I'd say it's set in retro-futuristic universe strongly reminiscent of Imperial Britain and the High Seas Adventure type of story (think: Master and Commander in space, but with equal parts space opera and Empire-like politics). If you're into that, you should enjoy it.
I've always enjoyed William's relatively straight-forward writing style; I find it helps me visualize the story and become immersed in it.
I thought the characters were well-drawn and their actions were quite consistent and believable given their back-stories.
Part of my brain nags me that no realistic far future universe could possibly be like that in The Accidental War, but that's because of the influence of other books I've recently read.
Within the specific type of story this is, I found it to be quite the page-turner.