To each their own.
Though I've always found playing a Space Marine to be quite fun. I suppose the Horus Heresy novels have really helped get me in to the subject more. There are some similarities to the 'band of brothers' feeling evoked by Only War. You read Flight of the Eisenstein you read about Death Guard Captain Garro and his "Honour Brother". It gives me idea that they're far more than killing machines that can't think past warfare.. usually.
The interplay between the group and their varying personalities and beliefs is where it's at outside of combat. They have to depend on each other, and the kind of bonds that can be forged while fighting with the Deathwatch are very strong. I want my White Scar to party with a Space Wolf!