I have a Kifaru Express and an E&E that I take on and off the Express depending on what I'm using it for. I love it, and the E&E is a must have and worth every penny, I sometimes use it by itself. Definitely not cheap, but it's probably the last pack I'll buy. It's a bit heavy, but as comfortable as any pack I've carried.
When I bought it, it was a toss up between the Express and the Marauder. I like the simplicity of the Marauded where one zipper opens up the whole bag. In the end I went with the Express because I thought it would raise less scrutiny when going through an airport. I use it as carry on and can keep most of my essentials with me.