In graduate school, I had the 1964 version of this book assigned in my research methods course. This was the book, then, that introduced me to the philosophy of science. I rather liked its bias toward emphasizing what researchers try to do rather than what philosophers "think" that they should do. My colleagues and I also appreciated his emphasis on the "autonomy of inquiry, (Page 3) ". . .the principle that the pursuit of truth is accountable to nothing and to no one not a part of that pursuit itself."
The book is, for a work on philosophy of science, lucidly written. Even after the passage of decades, it still reads pretty well.
This is a unique book on science and methodology. In places it is profound and invites you to think about topics for hours or days. In other places it is very difficult to follow and comes across as opaque. Rewarding but un-rateable, and definitively not an introduction to the topic but rather an advanced treatise to be engaged with over years.