I had my H-1B visa transferred between companies last week. Took seven business days, not months. My new company paid for premium processing, which is supposedly guaranteed to complete the process in 15 calendar days.

The process isn't complete though, correct? You have authorisation to work for your new employer, but the actual transfer is complete- that takes months.

Not that there's anything to worry about usually, it's just the kind of uncertainty that really isn't helpful when you're setting up a company.

Not correct ... as soon as the transfer goes through you can start work at the new company.

That's why I said:

"You have authorisation to work for your new employer"

But the actual transfer is not complete. You are not guaranteed to be able to continue your work. The chances of problems are small, but it does happen.