First off: if you've read the first two books, for goodness sake, YES, read Mockingjay. This isn't like Lost (which I never watched) where viewers eagerly awaited the ending only to be so disappointed they wish they never stated watching at all. While people might quibble about details, Mockingjay is definitely a worthwhile ending. And if you've been holding off on the series to see if the ending was any good, you may now safely commence reading.
At the end of Catching Fire, some of the surviving players have broken out of the arena, capitol rebels have demonstrated their intent, and District Twelve has been destroyed. The uprising (and corresponding smackdown) are both well underway. While all this appears to be Katniss's doing, really all she was trying to do was survive and save people she cared about. Rebellion was inevitable. Still, she's the proximate cause of the revolution. Now what? Will she step up as leader and encourage the rebellion? Will she try get everyone to cease hostilities? Or will she just nurse her wounds and hide?
The tone here is a little more grim (hard to imagine, I know!) but before it was just the contestants fighting for their lives -- now it's the whole nation. There's still occasional humor, a good amount of clever, interesting, horrifying developments, and numerous well-rounded secondary characters.
For the most part, everything progresses logically, though the initial premise is so absurd it's hard sometimes to gauge what's reasonable. In particular, some of the capital defense strategies are hard to fathom. But their entire rule was hard to fathom, so… yeah. My biggest criticism (and I seem to be the only one in this) is that I never understood President Snow's motivation. He just seems evil. Creepy disturbing clever evil, but still. Nothing deeper.
I won't say anything about the ending except this: once you finish the book, go back and read the first few chapters of The Hunger Games, and see how carefully that ending has been set up. (This is not a spoiler.) I was very impressed. The writing looks easy and effortless -- no fancy prose, no especially stunning plot twists -- but the whole arc shows serious forethought and skill.
As for the series as a whole: I thought it was very well written and quite enjoyable. But if you want a realistic science fiction novel THIS IS NOT IT. This is a greek myth made into a young adult novel. The superficial trappings of SF are used to create the setting, but the setting's purpose is solely to amplify the peril and the moral questions of the plot, not to extrapolate anything. I happen to think that's a perfectly reasonable use for science fiction, but there are lots of passionate readers who will not agree. If you want your SF realistic, please do yourself a favor and skip it.
Finally… I keep wondering how they're ever going to make a movie which is true to the books and yet not so loaded with graphic violence that it's unwatchable.