I too decided against having the vaccine for a few reasons:
1) It's a very new recommendation with not enough studies to support its validity as of yet. Up until recently, doctors would also recommend 'cocooning' too (i.e. vaccinating everyone in baby's close environment), but now new studies are challenging the value of this advice, so more and more doctors are now considering it unnecessary.
2) We live in Southern California. We have very long summers with hot weather often lasting 'till October/November, and baby is due any day now, so will be born in September. This means that he will be vaccinated for whopping cough in November at 2 months, before it gets cold enough here for any whopping cough epidemic to start/catch on. I'd definitely consider it much more seriously if we lived in a colder climate or if he was going to be born in the late fall or winter months.
3) Tied with #1, I was just unsure about any side effects and didn't feel like risking it. I was at somewhat increased risk for preterm delivery due to an irritable uterus (luckily that didn't happen and I'm still pregnant at almost 40 weeks now), so I didn't want to take any chances. Irrational, I know, but I couldn't shake the fear of something bad happening to me or baby due to the vaccine, so in the end I decided to turn it down and not get it.
I was told by my doctor that the best time to have the vaccine was between 28 and 36 weeks of pregnancy.