I usually use OO for any larger project (more than 1 file, 100 lines, but YMMV). It's useful when the project deals with complex data structures, there are several types of them, and each type can have many instances. I can't imagine we'd be ever able to build something like PML-TQ without objects.

(jeffa) Re: OOP is one of the nodes that Search returns for "OOP", it seems to summarize the advantages clearly.