I wasn't particularly familiar with John Murphy's work outside of the 28 Days Later theme and his Addagio, but I've just realized that I actually have several of his tunes suggested to me by Spotify, and therefore close to other things I listen to. I've also just rediscovered the 28 Weeks Later soundtrack, which is very atmospheric and aerial and which was actualy in my listening loop for a while, back in 2008. I really love this kind of stuff.
I'm less familiar with his work with Gunn on GOTG - these are films I've only seen once - and I understand that others weren't entouthiastic about his arrival on Superman... But honestly, I think he could have brought something really fresh and new, in the more electronica register.
I say “could” because he obviously went in a completely different direction with a variation on Williams' theme. To each his own, but I'm surprised that so many people hang on to it.
For my part, I feel it less like a tribute than a facility, and re-listening to it, I still can't shake off that “we didn't get the rights to the original theme” feeling.
Perhaps watching the film will put all this into perspective...