I have! I'm an ISR Certified Instructor and have been teaching in Arizona for 5 years. Now, I'm a little biased, but its an AMAZING program that has literally changed my life.
I enrolled my oldest when she was 10 months old because we had a pool that was not fenced. In AZ, drowning is the #1 accidental cause of death in children 4 & under. The summers here are horrible with sometimes multiple drowning or near-drowning stories weekly, many of which do not happen in the home, but at a friend's or relative's house, but that's another story altogether. Anyway, I was so amazed with my daughters lessons that I decided to quit my high-paid job in software sales and pursue becoming an instructor.
This is NOT like any traditional swimming lessons. First of all, the instructors undergo a 5 week in-water training program with a Master Instructor and study child psychology, behaviorism, physiology and psychology. I personally put in about 90 hours of study between both my in-water work and academics. As skinnydipper said, all children progress through different developmental stages at different rates. The lessons HAVE to be private because they are child-directed and based on the individual child's skill level. Private lessons are also so much safer for your child. I've aquired a few clients from a local swim school here because of children who slipped off the wall or off the lane line during a group lesson while the instructor was distracted with another child.
We start as young as 6 months old, but the goal at that age is strictly aquatic survival. The babies learn to roll on their back and float and wait for help. Once the child is over a year old, they can learn to hold their breath and swim underwater, then roll on their backs to float, rest & breathe, then flip back over and continue swimming. It really does work like you see in the video. I've trained more than 150 kids just like that. I have lots of pictures and underwater videos of my own 3 kids as well as my students doing what you see.
The problem with "water adjustment" lessons is that the child learns to become comfortable in the water before they have any skills. From birth, we send the message that water is fun. It IS fun, but it is not SAFE unless your child has the age appropriate skills to handle themselves in the water. Like Poolsean said, they are NOT going to be doing the butterfly (or freestyle for that matter) at age two. Most kids don't have the cross-coordinational skills for formal stroke work until they are at least 4, but they can still swim like fish as young as 1! All 3 of my kids did! Summers in our house are fun!!!
Anyway, most any parent who has been through ISR lessons will give you a glowing review. I have lots of parents who are references if anyone wants a first-hand opinion!!
Happy swimming!