My fiance got a $10k sign on bonus from a previous place of employment (a very reputable retail pharmacy), and it was split up over a 5 year period. But just to play devil's advocate...
His place of employment started to turn sour around year 3, and pharmacists and managers were being laid off, leaving him working severe OT without the help of pharmacy technicians or other employees. Customers were frequently complaining, and in FL, the RX pill problem is very bad, so dealing with irate customers, and robberies, etc... He wanted to leave, and even pointed out to the district manager, that pharmacists were being laid off left and right. The DM's response? "We own you for another 2 years, and if you leave now, you'll be paying back everything we've given you in bonuses."
If it's your dream job, congrats, and like the above poster said, the bonus is all gravy on the train. Just make sure that you know administration and management details, like if they're looking at a possible hiring freeze, or what the turn-over rate is, patient satisfaction, and so on.