Everyone over here on HN seems to be quite negative about the SABRE engine. While there's very little evidence for this engine being useful for orbital applications, it is instead a very compelling idea for a Concorde and/or SR-71 replacement. Scramjet engines don't seem to be happening anytime soon (I just listened to a Queensland University researcher saying that their research may yield an actual engine in 4-5 decades). This idea of pre-cooling the incoming air seems closer to reality.