Looking back, I think the key to it is being realistic in your expectations, understanding what you need/want from school, and how much effort you will be happy making trying to integrate with the french. If you only plan on being in Pau for a year or two, it might not be worth the hassle of going to a French school.
ISB is really transient - kids often spend just a few years there before moving on - either back to their home country or a french school. So it might be worth starting at ISB and think about moving later once you are more settled. Or having it as a back up if french school doesn't work out.
The parents who were English and moved more permanently to Pau seemed less happy with ISB. Especially when they had older kids and could compare with 'good schools in UK'. They often moved to french schools, or quite often just moved back to England.
Parents who had experience of older kids in French system often liked ISB, because it was less rigid.
English/French couples liked ISB because it was a good opportunity for the kids to learn English. But they didn't like the fees.
Families there on short term contracts were happy because they knew they would move on soon, and following the English curriculum was good.
Whether you chose ISB or not - Pau is a great place to live. Enough English families that you don't feel isolated, but it doesn't feel like 'Little England', unlike other parts of south west France.