I had a problem where I needed many-to-many relationships between custom records (or entities and custom records.)   I work for a hosting company, and I needed to keep track of which servers a customer was hosted on, as well as which customers were hosted on a server:

[Customers]<---->[Servers]

NetSuite's built-in parent-child relationships are not well suited to this: you can either set up a 1-1 parent child relationship or you can set up a 1-to-many parent child relationship with "built-in" child fields (adding a multi-select field to the parent record that refers to the children.)  Many-to-many would seem to require two separate links from the child to the parent and from the parent to the child, which would require manually keeping the data for each of the two links up-to-date every time you made a change: