How good would you say this book is?
I fancy getting back into spongeball magic.

I would say that when it came out in the 70's it was a fantastic resource but today it isn't worth the prices that the book usually goes for. When you consider all the other options for learning there is better value elsewhere.

I remember a few years back it was selling to collectors for around $300!

It does contain some classic routines you won't find today.

As stated above, these days you can assemble a much more comprehensive set of materials for a lot less money.