I have to admit that I didn't read this entire omnibus. I actually stopped reading about twenty or thirty pages into Orcslayer (the second novel), which I will explain below.
As for the first novel, Giantslayer, which is written by original author King... it is pretty good. At the very least, it expands the Warhammer universe appropriately and has a few very epic and interesting scenarios. Unfortunately, it did seem like King was getting a bit bored with the series considering that some of the prose and dialog used throughout the novel is incredibly repetitive (even more so than in his other novels). Based on Giantslayer's ending, I think King either knew for sure that he wasn't going to continue the series himself or he at least had plans to change thing up quite a bit in regard to the setting and supporting characters.
Orcslayer is, of course, the first novel in the series written by a different author (Nathan Long). I have no reason to think Long is a poor writer in his own way, but I can tell you with absolute certainty that he doesn't stay faithful to the Gotrek & Felix characters or Warhammer universe (which very much contradicts his statements in the book's forward). Instead of explaining this in full detail, something that can be found in other reviews of Long's Gotrek & Felix novels, I'll offer an analogy: reading King's Giantslayer and then diving directly into Long's Orcslayer is like watching the DVD version of Lord of the Rings: Return of the King and then immediately switching to a Hercules: The Legendary Journeys made for TV movie starring Kevin Sorbo. I'll let you figure out what I mean by that.
I plan to return to this series and attempt to continue with Long's novels after I've let this omnibus sit on my shelf for a few months. I'm hoping that some time spent away from the series will allow me to look past the way Long changes Gotrek into a cartoon character and leaps backward with Felix's development as a character.