I Bf all of mine
ds1 only to 10 weeks, then i went back to school, couldn't keep up with expressing the amount he needed when i was gone, tried mixed feeding, but it didn't agree with him, and so had to go on to formula.
dd1 - 13 months
ds2 17 months
ds 3 13 months (i was 7 months pg with dd2 at the time, and he couldn't get comfy any more)
and dd2 13 months - she bit me one morning, and i yelped, and she flat refused to try ever again!
will definitely bf this one - Im too lazy to FF 
you have to buy the formula, buy the bottles, make the bottles, sterilise the bottles, wash the bottles, warm the bottles.......
all that instead of getting the boob out - no contest really! 
however - i feel it is an extremely personal choice, and definitely NOT something one should ever be made to feel guilty for doing/not doing.