From my experience, "The Last Person Got" is right about 75% of the time. However, I've also seen reports on other forums of people furious that they didn't get what the last person received.
I've also seen hotels in Vegas that show "last person got" that I know are wrong by deduction. For example, Hotwire shows that the last person got Vdara for a hotel that has a casino (Vdara doesn't have a casino).
So I would say that the Last Person Got is not fool proof, but does work sometimes. Use it in conjunction with other sites like
HotelDealsRevealed.com to figure out what hotel you're getting.
Until you actually book, it's ALWAYS a risk.