I've got a skinny six year old who weighs under 50 and he's nothing but knees and elbows. I also have an 8 year old that's 81 lbs and built like a brick s&^thouse. The waist band around the speed sac accommodates both (although is more acclimated to fit my bigger boy).
The outside of the speed sac is durable, like a tough rubber mat that you'd put in your pickup. The velcro for the belt does it's job perfectly. The loop joining the bag and belt is big and solid, and the hook mechanism to hold the three 10 lb sand sacks is more than adequate. Nothing is falling out of this.
Strap the belt on you, walk up to the line, then go. For my boys, I use 20 lbs for my 8 year old and either 10 lbs or nothing for my youngest. The bag is heavy enough for a little sprout that you don't necessarily needs lbs of resistance.
Like other SKLZ products, this one comes with a workout poster. One one side, a poster with recommended drills for only the speed sack (number of reps, distance, etc) in a 6 week format. On the other size, a 6 week workout program that combines the speed sac with other SKLZ products (such as the speed hurdles, quick ladder, viper 360, etc.), and both have beginners/advanced adjustments.
It also comes with a DVD, but the DVD isn't just for the speed sac. The DVD gives demonstration videos for most of its products. Each video clip is around 20-30 seconds in length (speed sac has three videos, if I remember correctly). Each video demonstrates regular speed and slow motions with different angles so you can see and teach proper form.
As for its effectiveness, that'll remain to be seen. But in football, your first step is the most important step. If you can get off the line quickly and keep improving that quickness, you'll improve your game. On the line, first man off the ball should win the battle. This product improves first-step speed and overall speed. It will help you improve your stride, your body lean, and your leg strength.
The three bags, which you have to fill with sand ($4 for a 50 lb bag of sand at OSH), have a handle on one side of the bag so you can carry then down by your side while you're performing walking lunges. Great way to wrap up some 5 x 15 yd runs.