It sounds reasonable - why would I want to pay for something I'm not using, right? Well, there is a catch - you pretty much signing in for a lottery. And I "won". Twice. First time it was a small fish - unreasonable activity in database set me up for around $25 out of nowhere. Not a big deal, I just turned it off - this database was used for testing and not needed to be online all the time. A bit inconvenient but manageable. I should have canceled pay-as-you-go right there because I only needed subscription for dev testing, and free option would do. But decided against it, just to have some flexibility in the future.
This costed me $10-15 a month, nothing to worry about. Until it went to $150. Ten times up for no reason. Just a testing site with no users or processes mining bitcoins in the background. Wow. I mean, not the end of the world - but what if I was running small business? With operational expenses going from $1,000 to $10,000 overnight? No good.
But hey, that's MS, not some cheap shmucks running a cloud in their basement - they can figure it out and get things back to sanity quickly. Well, It's over a week since they start looking into it, and doesn't feel like any progress was made at all. Won't be surprised if they end up with nothing but suggestions how to avoid it in the future. Although I already have one - just stay the hell away from that pay-as-you-go option.
Update: To their credit, MS have agreed that this charge was a fluke and issued a refund, so all well that ends well. Still, I'm really not a fan of this option at all. It basically puts all the risk on your shoulders. So if you must - do your research, keep constant track on any activities, probably put a limit where you want service to stop. I mean, who knows what one good DDOS attack can do to the monthly bill.