Since I've used Mag-10, I'll chime in... I went on a 6-7 week straight cycle and saw little in negative sides, but my strength skyrocketed!! And my weight was up a good 15 pounds (some of which was likely water weight).
Some of the people who have problems could have abused it... or as with anything, different people react differently. Some are more prone to male-pattern baldness, some get acne easier, etc.... If you see sides you don't like, quit taking it and never use it again.