I despise people who go on and on about how they feel the speed-bag is useless. Often citing exercises that don't work at all in the same way a speed bag does. I'd be willing to wager they don't use the speed bag properly and thus, don't feel any benefit. But when done right, it works on everything from punching endurance, to rhythm, to breathing pace, to foot movement while punching, to the ability to change pace through the course of a round of Fighting.
Speed bag is typically done at the end of a workout as well, when arm fatigue has already set in. It shouldn't be hit robotically either. Here's a robot doing it:
YouTube - John Duddy Workout 3 of 3
Back all stiff, no fluid arm-movement, no head-movement, no foot movement.
Here's a guy who can box working the speed bag:
YouTube - Bojado on the Speedbag
Double-end bag? Here, you're looking for about 3:52 or so:
YouTube - shane mosley training double end bag
Now I've seen people post here that mitts should be able to replace both of these apparatus. I highly disagree. Anyone ever seen someone hit mitts at the pace Bojado works the speed bag and Mosley works the double-end at full speed? It's not very often both the catcher and thrower of punches can keep that up.