For some reason, I never got around to reading this when I was young. When I found it as a free download, I thought I might as well do it now.
Generally, I agree with many of the other commenters. The book isn't great as literature, but as a propaganda piece, it's pretty effective. As literature, much of the book is overly dramatic and sentimental. The characters are two-dimensional, and the story wraps up pretty neatly. The writing is decent. There's never a moment, though, in which you forget that the story is the carrier for the message, rather than the other way around.
On the propagandist front, Stowe makes a very good effort at showing several sides of the key issues: religion and slavery. There's no question which side she is on, but there are nuances even among the slave owners, and she castigates Northerners just as thoroughly. All in all, it's a very effective piece of anti-slavery propaganda, often through fairly direct argument to the reader. If anything, she's light on the horrible effects of the slave trade, focusing most often on emotional disruption.
Unfortunately, while consciously fighting the evil of slavery, Stowe often relies heavily on stereotype (blacks are naturally generous, credulous, and good cooks; women are emotional and not naturally suited to business). That's largely a sign of the time, of course, and she was much more progressive than most, so it doesn't grate as much as it might otherwise.
Equally troubling is the inescapable religious message. Stowe is clearly a Christian, and the solution to just about everything in the world is just for people to accept Christ into their lives. Do that, and you can die happy, even as a beaten, tortured man who's been ripped from his own family and several others. Conversely, in the final chapter - a direct appeal to readers - she suggests that only Christian Americans have a responsibility to do anything about slavery; atheists apparently get a free pass. To her credit, Stowe offers a reasonably balanced portrait of an agnostic in the book, but her underlying message about Christianity is hammered in, page after page. She never considers that 'Africans' might have their own beliefs, but that's pretty much true for everyone else she describes as well.
All in all, worth reading for its historical value as an argument against slavery at a crucial time in the United States, if not as a work of dramatic literature.