In my opinion the best of the series by a good margin. I'm not always a Moon fan - some of her other series seem to me too narrowly focused on a female teenage market (tiresome family, handsome suitors, underrated heroines and so on) but the Vatta series has a good balance of technical interest and nuanced characters: nobody's perfect, and only the pirate chief is undeveloped.
This volume has several interesting twists: new developments in ansible technology, the daredevil romantic pilot from a schizophrenic culture shifting personality at a crucial moment, divisions and rivalries imperilling the alliance, and a satisfying final space battle. There are also quite a few loose ends (e.g. the shadowy adviser in volume 4 turns out to be nobody in particular): as in real life, not every detail is significant and you have to guess which will matter.
It could have done with a little more editing - the occasional repetition of phrasing, the uneven character of Rafe (sometimes a daredevil Rhett Butler type, sometimes just a foil for heroine Ky).
So should you buy it? To enjoy it you really need to read the full series (and I do recommend that if you want light fast-paced SF with some three-dimensional characters), but if you've got to volume 4 I'd certainly recommend getting this to round it off.