Make sure to not scrape too much with the credit card (or old Oberweis points card) or it's guaranteed "razor burn" -- ouch! If things don't come off easily enough, wait a few more minutes for the hair to dissolve! No ingrowns for me, and I was impressed with how things turned out, although I think I prefer the super-smooth, powdery feel of a post-wax (and a wax removes ALL the fine hairs, too). For $1.77 a can, though, it sure beats paying ~$50 per Brazilian!
(I planned for 15 minutes of Magic Powder time, but overall it took me about 45 minutes, with all the waiting and doubting and then scraping and rinsing.)