The longer the novel, the more wary one should be of it. Reading a novel takes time. The longer a novel, the more time it demands. One would hope the longer novel would then provide a greater payoff - a more emotionally rich experience, a more satisfying plot, a sounder resolution - but this is rarely the case. I would therefore usually hesitate to unequivocally recommend a long novel, let alone a really, really long novel but that's what I'm about to do.
I've spent a fair chunk of this year reading the first two thirds of Neal Stephenson's The Baroque Cycle. A science-fictiony counter-historical story set in the latter half of the early modern period (for those of you whose spouses aren't historians that's around 1660 to the 1800s), combining swash-buckling pirate adventuring, the foundations of modern science, and a pretty detailed examination of the birth of market capitalism. Stephenson draws many subtle parallels with the modern world - the fragility of the markets, the dangers of a credit bubble, the birth of the information age - while weaving the story through historical events and real people without ever seeming forced or clever-clever, or even just sluggish. On the contrary, it whips and bucks along, and all you can do is be carried with it. The scope of the whole thing is pretty breathtaking, and I found it absolutely gripping.
I'll be starting on the final volume, The System Of The World, tomorrow and I'm really looking forward to it.