My personal "Disappointment of the Month" award goes to "Monstrous Regiment".
Not only because it's a bad cross between comedy of errors and The Lost Generation's war novel. Not only because the central intrigue is grossly overdone (in the end you begin to suspect that Sam Vimes is also, you know, involved). Not only because many juicy promises are given throughout the plot and none of them is really delivered.
This is all so very important, but the worst part was that I couldn't read this crap with a straight face after, halfway through the book, I recalled Fred Reed's masterpiece about General Claudia: