I think it is an exceptional form of cardio.If you are already doing rounds, then you are on the right track.Try to build up to 5 3-minute rounds with only 30 seconds of rest between each round on both the
heavy bag and the speed bag.
You can also start integrating active rest into the periods between rounds whether they are 30 or 60 seconds.Instead of resting completely, do a set of lunges or jumping jacks between rounds.This will definitely increase the intensity of your workouts.
Boxing trainer
Ross Enamait has highly advocated the use of punch-out drills for improving conditioning.For these, you would use just the jab and straight rear hand aka the 1-2.Throw the 1-2 as fast and as hard as you can for 30 seconds, rest between 30-60 seconds (depending on your recovery ability) and repeat for up to 10 intervals each.These are fantastic drills.They make a great workout for when you are short on time or just want to add a little something to your heavy bag work.
I'm not too big a fan of the double-end bag, but it is effective for hand-eye coordination and defensive work.