I have been to three due to conditions of my funding. My experience is that they are a mixed bag! Def a chance to get to know people in your field and actually tenuously related to your field. Some of the workshops/talks are good, some okay, some were total rubbish. It all depends on how the person delivering it have been briefed. Did I learn anything from them? Eh, not really I don't think! I would have preferred if they had been of a more applied nature but I guess that could be just me? Sometimes I find the endless theorising a bit self-indulgent. I think some of the presenters and facilitators ignore the word 'school' and use them as an opportunity to give yet another presentation.
I sound a bit negative - I don't mean to be and hope I haven't put you off. I have a couple of colleagues who went to a few on the Continent and they reported that they were very worth while. Probably it depends on the programme, the calibre of who is involved and the brief those involved have been given.