The three books (Ship of Magic, The Mad Ship and Ship of Destiny) contain, as their sheer size would suggest, a sprawling tale concerning, well, ships and the people who own them. Specifically the semi-sentient liveships, sailing ships with animate figureheads, which are owned exclusively by the capital-T Traders of Bingtown, a coastal dot on the map of Hobb's previous trilogy (Assassin's Apprentice and sequels), now expanded into a port town run by the hereditary semi-aristocracy of the Traders, and famed for both the liveships (which are faster and hardier than any other ships around) and the marvelous goods which they carry.
The story is told in a number of separate threads, most of them concerning the fate of the Vestrit family, Traders from Bingtown, with additional threads following the exploits of a pirate named Kennit who wants to make himself a King, and a tangle of sea serpents, whose role in the story doesn't become clear until the middle of the second book.
At the start, the various Vestrits fall into well-worn stock types: there's Althea, the Tomboy, who wants to be a sailor, and run the family liveship; her mother Ronica, the Matriarch Trying to Preserve the Family Fortunes; Althea's sister Keffria, the Mousy Wife of Kyle Haven, the Male Pig, who is given charge of the ship instead of Althea, and promptly makes a hash of everything by going into the slave trade; Kyle's and Keffria's son Wintrow, the Apprentice Priest Dragged Into the Real World; and Wintrow's sister Malta, the Flighty Adolescent Girl whose chief concern is for the social scene in Bingtown. About halfway through the first book, the pieces are all in place, and it seems clear that Important Lessons will be Learned by All.
Indeed, you can even tell what the Important Lessons are going to be, and sure enough, most of them are Learned by the appropriate characters before the story ends. And, of course, the characters are all put through the metaphorical wringer six or seven times before the end of the series.
Plot-wise, there's nothing all that groundbreaking going on here. It's a little complicated, and there are a lot of separate threads to the plot, but it's all pretty standard stuff. The setting is quite nice, though, with a few clever touches thrown in. It feels well-thought-out, and there's a nice air of mystery about a few of the settings.
These books are only for those who are comfortable with Honkin' Big Trilogies, though. The plot has two or three threads too many for its own good, so that while none of the storylines are obviously padded out, the sheer number of distinct plot threads gets hard to follow. On top of that, Hobb's tendency to heap miseries on sympatheic characters (as in the Assassin books) makes parts of the books quite difficult to read. She does a nice job of fleshing out the characters, though- the villains have comprehensible motives for their actions, and the heroes have their flaws.
As long as you're comfortable with the scale of the books, these are a good read until late in the third book. At which point, a couple of ill-advised decisions are made. The more minor of these feels like a heavy-handed attempt to counteract the excess sympathy built up for one of the "bad" characters by inflicting needless trauma on one of the "good" characters. The bigger flaw is the ending, which is a little too pat, and feels almost like a reaction to the downer ending of the Assassin trilogy (there's a slight overlap between the two stories in the third book, which feels a bit forced if you notice it among all the other things going on...).
If you enjoy Whopping Huge Fantasy, these are worth a read. The books are written and plotted well enough, they're justbig, which means they won't be to the taste of some readers.
I can't help wishing that Hobb (whose real name may or may not be Megan Lindholm, though she's certainly written under that name) would write something a bit, well, smaller. I loved Wizard of the Pigeons, and would like to see something more in that vein.