Right, well, before buying this book I spent a good while reading all of the other reviews on here and on other websites, simply because views on it are so massively opposed. Final Crisis really is the 'Marmite' of the DCU.
I will say this to anyone tempted to buy this book: read Crisis On Infinite Earths, Infinite Crisis and 52 first, they will help (or you could just Wiki them) and they're all amazing anyway. Of the three Crisis stories, this one is certainly the most complex and interesting.
The reason why I have not given Final Crisis five stars is because the writing often comes across as needlessly convoluted and, thus, a little arrogant, as if Morrison wanted to confuse the reader for the fun of it. This is especially true in the Superman Beyond issues half way through. Despite this confusion, the story is not all that complicated, but it does jump from one time and one location to another fairly frequently with little explanation and the major 'shock factor' turning points and revelations, the moments which are plot-changing and world-changing are incredibly dampened and downplayed; if this was a film, these moments would have no slow motion, no classical music and no gasps or loud and agonising screams. This is a very interesting approach to storytelling, making the entire book seem like one huge crisis, rather than several small ones, which is exactly what it is.
On the art side, J.G. Jones certainly delivers. The man behind the beautiful artwork of Mark Millar's Wanted has shown, once again, how to create a dark and gritty world that's also full of hope. Simply beautiful.
Final Crisis certainly has its good points and bad points but, overall, it's an exciting and excellent read and I recommend it to anyone who wants to read an ambitious story and experience an interesting (understatement) period in the DC Universe