We thought the guided tour was very informative and worthwhile. The guide was professional and well-informed, her English was excellent. The tour was thorough and I think took at least three hours including Birkenau. We didn't have a choice about it as we were in a large tour group, but I had no regrets.
I think they require the guide service during peak hours to handle the many visitors. You may be wise to start earlier if you want to tackle the sites on your own (don't miss Birkenau, which doesn't take as long). I recall a lot of signs and labels, but I don't recall whether they were in English -- probably were, along with other languages. One thing I recall is that the guide referred repeatedly to "the Nazis" rather than "the Germans," doubtless in recognition of German visitors' sensitivities and the fact that Germany has faced and repudiated that chapter in its history, perhaps more effectively than most other nations.