That Arsenal were allowed to win five of the last eight indicates the decreasing status of the FA Cup IMO. The semi-finals of the FA Cup often collide a little bit with the CL semi finals and teams are going to priortiise the latter.
I never bet on tournament winners but 40/1 is probably decent in the sense that I think the chances are actually a little bit better (30-35/1?) but still very likely a chance of just wasting money.
Arsenal 12/1 is poor value.
Tonight I will be betting on City vs Swindon, probably under 4.5...