I haven't either. But, I work directly for customers, and I've seen entire departments wiped out and replaced with H1B visa holders. They are large telecommunications and financial companies that you would recognize.
I can also tell you that as a group, the quality is much lower. While there are good people, there are many that simply don't have the problem-solving and analytical skills that are necessary to be effective.
If I had been the hiring manager, I wouldn't have hired them. If I had interviewed them, I would have explicitly said: no, they aren't qualified.
I'll leave it to you to guess why they were hired anyway.