Anyone used advanced BOMS? Can you have multiple revisions active at the same time and when creating a work order select what revision the BOM has?

Definitely. 🙂

Messing around with it in the sandbox it seems like you have to have a start date and can't have them overlap with another BOM for the same item

I think you can have one effective revision per BOM for the same item, though you can have multiple BOMs for one item.

Revisions dates cannot overlap if you create multiple Revs.

Yup you need to enter at least the effective end date of one of the revisions so the other one could start one day after such end date (no overlapping then). In the WO, the default revision will be based on the WO date against the effectivity dates of the revisions but you can still select other revisions, if necessary.

Do you know if you import revisions via CSV?

you can import BOM and BOM Revisions. though the last time I went through the process a few months ago it was a little painful attempting to perform updates. if you are creating revisions via import it is a little easier