When I moved from Canada to the Bay Area, I accepted my job offer on a Friday evening, organized to get original transcripts from my university on Tuesday morning, FedEx'd them overnight to the law firm and they arrived on Wednesday. I received my TN-1 package (a thick, completely opaque set of forms, documents, letters and all sorts of other legal shenanigans) on Friday evening. I then bought my ticket to leave Canada the following Friday. All together it was 2 weeks from acceptance to arrival in the US.
Not every experience is the same, and no doubt it would have been far harder had my company not pipelined my immigration through a very prominent and large law firm.
That being said, the fact that I can be denied entry in the US at any time scares me. It scares my company. It's no secret it's expensive to hire, and even more expensive to hire new talent to replace talent that cannot enter the country. I say this as a young, educated professional with no criminal history or ties. I shouldn't have to worry about this.