We had one recently. I've also had them before in this and other schools. It was fine. If you're under 45, the inspectors are basically like the teachers you had when you were at school. The buzzword this time from the inspectors seemed to be differentiation.
Our school made us jump through a lot of hoops and prepare a great deal of stuff that ISI hadn't asked for - and even stated explicitly on their website that they didn't want - and therefore never looked at, which caused a lot of ill feeling internally (not ISI's fault).
I find it all a bit of a sham as the inspectors have a distinct air of "being on a jolly" about them (all their expenses are paid by the hosting school) and the school has to be a bit like a show home with the inspectors smelling only fresh paint everywhere like the queen. Also the students and staff who speak to the inspectors are chosen by the school so there's no chance of them hearing any criticism.