Dragonfly begins almost straight after the events of Empire - we do get a brief recap, but if you haven't read the first novel, you'll get lost fast. Thalric is back in Collegium, and so is Stenwold. If you've read Empire, you can probably guess the idea: Stenwold plans to warn of the Wasps' invasion (for the umpteenth time).
Tark is beseiged, and the Ant city is visited by Totho and Salma - who, of course, has a ulterior motive. At war, however, their path becomes less clear - and Totho begins to find tougher choices in his role as an artificer. This was a major improvement, in fact: Totho was a character who didn't hold much interest in the first novel, and that he gets development and some harder choices, rather than (boring) angst, improves him immensely.
The series also improves with a lot more action. Although Empire in Black and Gold seemed slightly padded with the amount of journeying, Dragonfly strikes a better balance. it's fast paced, with more battles, and more innovations (even if some are more than a trifle... unlikely). We've also got more of my favourite activity: intrigue. The Spiderlands were an area discussed frequently in the first novel, with their complex manipulations - 'dances'. To find them getting involved... Well, it's evident that not all is as it seems. After all, when is it? In books, never. (If the question is asked on the blurb, the answer cannot be 'yes; yes, it is'. This is a rule.)
While there are some good moments in Empire, Dragonfly Falling is the first novel where you can truly see some of what TvTropes would call 'Crowning Moments of Awesome'. I won't point any fingers - spoilers, people! - but a few seiges come to mind. If you enjoyed Empire in Black and Gold, but worry about its sequel getting 'middle book syndrome', read Dragonfly Falling anyway. It's a much, much better novel, and the series' prospects increase immensely with its ending - and its character development. There are still flaws, of course, including the tendancy to characterise by kinden. But overall, this is a book which makes the promised epic truly enjoyable. This is well worth your time.