I'd say about an 6.5/10. It is a very dull, nauseous pain that lasts for minutes and honestly makes you want to curl up and wince around in pain.
However, I wouldn't say it's anywhere near the worst pain I've experienced. I've had my dick smashed before and that feels Much worse, it's like a hot nail being driven through your crotch and it really will incapacitate me for some minutes. My ribs are also a terrible pain when they break, but the worst is the pain lasts for months after its all over.