Probably more useful for understanding Popper than for understanding the people Popper writes about. Not that understanding Popper isn't worthwhile, but this should definitely be supplemented by other sources if you want to understand any of the historical figures Popper talks about. Admittedly, I'm having trouble thinking of a really good history of philosophy without an agenda of some kind; perhaps the best way to go is to read things like this (or Russell's A History of Western Philosophy, another notoriously inaccurate work mentioned elsewhere in the thread) and remember to always take it with many grains of salt and do further research on anything that seems important.

The Porverty of Historicism is another book. BTW, the overall approach, making theories restrictive enough.