It was a good epsiode, and explained a lot. I'm of the opinion that Jack hasn't switched sides, but is either just acting to get a good look around, or has simply accepted that he's stuck there and so is making the best of it.
That said, they didn't show the preview for next week on Sky, so I might be missing something.
Two things really bugged me about this episode: The first, and lesser of the two was when Claire kicked Charlie out. Here we go again, I thought.
However, the thing that really bugged me was that Sayid seemed to just accept that Locke had the C4 with him. At the very least, he should have been asking a bunch of very pointed questions and not going on until he got answers. This is at least twice now Locke has deliberately sabotaged a potential means for the survivors to contact the outside world. And, in fact, the first time involved him knocking Sayid out in the first season.
Oh, about the sonic fence, and why there aren't masses of dead animals... won't that just be learned behaviour?