I've got one mounted on my MP-15 Sport and I love it. I've also got one installed on one of my AKs. Some of the best money I've spent on upgrades for my rifles. They are a blast to use (pun intended!)
...Erratic, uncontrollable, & goes against all principles of marksmanship? Interesting observations from someone who probably has never fired one or even seen one fired in person! It's a piece of cake to fire controlled two/three/four round bursts and keep the rounds where they need to be. Or, you can cut loose and dump the whole mag. Brings a smile to my face every single time!
BTW, I served 25 years on active duty and I never got the opportunity to fire anything full auto. Maybe this is my way of compensating for missing out on that opportunity?
Bottom line, buy one if you want to because they are the most fun you can have with your clothes on! I had to file a small bit of plastic off the inside of the stock because of the bowed shape of the trigger guard. After that was taken care of, she ran fine.