Bran Mak Morn is the last king of the Picts and will do whatever it takes to help his people. That's all you need to know going in.
I liked this omnibus about as much as the Kull one I read at the beginning of the year but not as much as Solomon Kane and definitely not as much as Conan. My favorite story in it is Worms of the Earth, where Bran makes a deal with some Lovecraftian beasties.
I enjoyed it and I'm glad I read it but I wouldn't consider Bran an essential read for Howard fans.