No, it's definitely NOT worth buying, i saw it at the cinemas and i thought it was terrible. Probably one of the worst films i've endured this year (along with 'Aliens Vs Predator: Requiem'). Avoid it like the zombie plague.
Oh that movie was terrible also and a bloody waste of time, just glad I borrowed it off someone and didn't waste money on it! makes the first one look amazing.
As for Diary of the Dead, I didn't think it was one of the worst of the year, had its moments, it was just a very big disappointment, especially from Romero.