"Are Winsor and Newtion Series 7 brushes worth the expense?"
Purely subjective of course, but in my opinion, no.
They are exquisit, hand-made sable brushes, and like all pure sable brushes are intended primarly for watercolor. In fact we would nomrally say that using them with the ever more popular acrylics will just ruin them in short order... BUT these days with the Andrea/Vallejo technique of "very thin" (watered down) paint, you are getting close to watercolor and can really benefit from the flow control afforded by sable.
At roughly 1/2, even 1/3 the price, but still a most premium product, I've settled on the Winsor & Newton "Sceptre Gold II" which is a sable/synthetic blend so it gives you the control of sable when doing the super thin acrylics, but still holds up a little better in the long run.