We have taken dd's out of school twice during term time.
DH is allocated his holiday and has to swap with other people, he is usually able to do this and get time during the school holidays, however one year he was only able to swap for the last 2 weeks in July. We had planned a trip to Florida that year, therefore we decided to take the dd's out for the last week of term, I don't believe they missed out on much education as that week is usually spent winding down and playing games, both dd's were in primary school at the time.
The other time was one Friday to attend grandfathers 70th Birthday party in Ireland.
DD1 is starting GCSE's now so would not consider taking dd's out of school during term time, however in earlier years I don't think missing the odd week is that big a deal, as long as they are not behind and their attendance is generally good.
We are fortunate that we can afford to go away every year during school holidays, so we wouldn't take dc's out purely to get a cheaper holiday, but if our financial situation was different and we could only afford a term time holiday then we would consider taking them out of school for a week.
I also believe that children can learn a great deal on some holidays, for example, we took our children to Kenya, the amount they learnt in those 2 weeks was far more than they could learn at school in two weeks.