The book works very well the show just how bad Stalin was by using Napoleon, his animal equivalent. It, rather cleverly, shows how Stalin was no better, and in fact probably worse, than the Tsars he replaced, or how the pigs were no better than the humans they replaced (as it goes in the book). There's a nice mixture of allegory and very dry humour, which make the book rather enjoyable to read.
Having said that, the book does have it's bad points. The biggest problem, in my opinion, is that Orwell's writing style for this book is very boring, there's little description or emotion. Although, as the book's purpose is the serve as an allegory, I suppose that description or emotion aren't actually required of it, but still I think it seems rather lacking in this area and that made it slightly less fun to read for me. Also, I think that Orwell is slightly to generous to Lenin with his equivalent character Old Major he is shown as practically flawless. While Lenin was MUCH better than Stalin, he still did a few bad things during his reign.
On the whole this was a good book, I'd rate it at 7/10.
(Buy it here: http://www.amazon.co.uk/Animal-Farm-Fairy-George-Orwell/dp/0141036133/ref=sr_1_1?ie=UTF8&qid=1313427790&sr=8-1)