I first ran across Garth Nix in a public library, in the shape of his Abhorsen series. I thought it was great, and was surprised to find that other people already knew about it. In any case, when I bought a new Nook, I ended up with a 'free' credit to spend, and I looked around for some cheap reads. This book was one of them.
I liked the stories. The characters are engaging, the setup is straightforward, and the storytelling is smooth. Still, I didn't give the book a really high rating. Why? Largely, it's because all this seems so familiar. It's Fafhrd and the Grey Mouser with a slight change of costume. Even Hereward and Fitz' task is given to them by mysterious body eerily similar to Fafhrd and the Mouser's sorcerous mentors.
It's very possible that Nix is a better writer than Leiber was. His stories are certainly warmer and more personal, if less given to wry humor. But these stories feel very much like an emulation - for all I know, a deliberate one. If so, well done. But there's no acknowledgment in the book, and the stories simply don't have the originality that would induce me to score the book higher.
Overall, harmless fun, and a nice memorial to Leiber's characters, whether intentional or not.