I needn't have worried.
Prince Caspian is, if anything, an even better adaptation than the prior The Lion, the Witch, and the Wardrobe. The parts which could be boring and tedious to do on-screen are compressed -- in my opinion, masterfully so -- to a few evocative scenes and dialogue which get across their essence. For example, the book contains a wonderfully written sequence of events where we first meet Caspian, follow his childhood, see how Master Cornelius comes to him and teaches him, and then eventually leads up to the birth of Miraz' son and Caspian's flight.
This sequence, fun though it is to read, is mostly talking, almost no visual action, and would be death to film. Instead, we START with the birth of Miraz' son and Caspian's flight from the castle, all done in such a way that we get the essential facts about what is going on RIGHT THERE, and other essential details are brought out painlessly later. This is, in my opinion, nothing short of brilliant. Similarly, the chapters involving the Pevensies' entry into and rediscovery of Narnia, and long way to meet up with Caspian, are compressed almost painlessly.
The battle scene which was partially visible in the trailers, involving Griffons, which didn't look anything like what I remembered from the book (having just recently read it)? Turned out to be a brilliant demonstration of show, don't tell. It's clear that it would take Miraz TIME to mobilize his forces and ready for war, so it makes sense that a good commander would try for a pre-emptive strike, especially with special forces advantages (i.e., air elements, special infiltration troops, etc.).
The only real negative of the entire movie was that they introduced far too much tension between Caspian and Peter. Yes, I could see some of it, but Peter had gotten over most of his "issues" in the first book. I think he should've gotten over it and apologized to Caspian earlier.
The scenes we got momentary clips of, showing the White Witch (which I REALLY worried about)? Just a slight extension and more dramatic version of the sequence in which Caspian is offered the chance to summon back the "White Lady", as the nastier Narnians call her. Well-done -- not identical to the book, but for a movie I think even more effective. It also allows Tilda Swenson a chance to reprise her magnificent performance as Jadis.
The Chronicles of Narnia naturally get away with something that I wouldn't put up with in just any story: a true Deus Ex Machina. Narnia, being Christian allegory and directly focused on the aspect of "succeed, but with faith in God", requires that our heroes BOTH expend personal effort to succeed, AND also place their fate in the hands of God -- Aslan -- through faith. So the clever plans of our heroes cannot -- by themselves -- succeed. There must be both human effort and divine intervention. This movie captures that balance very well. The Narnian strategies work well, and are clearly executed carefully -- and are necessary to buy the time for Aslan to act. At the same time, it's clear that without Aslan, the Narnians aren't going to win this one.
(In this sense, Narnia and Lord of the Rings are extremely thematically similar. Sauron isn't defeated by the plans and schemes of Men or even Maia, but by pure providence: the fact that Gollum was spared earlier allows him to be the instrument of the Ring's destruction.)
Rumors I'd heard of a Caspian-Susan romance were greatly exaggerated, and the small amount of that shown was actually extremely appropriate (and symbolic of the fact that at the end, Aslan says both she and Peter are too old to return to Narnia, as Narnia is not their world and they cannot remain there). And I doubt many people could fault Susan, because Prince Caspian's a very VERY fine figure of a young prince, resolute, handsome, heroic, and romantic. And he's a nice guy, too.
I'm now cautiously optimistic that they'll make The Voyage of the Dawn Treader well, too.
So in short, a wonderful family movie well worth seeing.