The results after repair are dependant on how bad the crack is and where it is on the headstock. Did the headstock snap off completely or just crack, but not fall off? If the repair is done correctly, it should be fine and the guitar will likely never break in the same spot. In most cases, the repaired joint is very strong, so you probably won't experience any loss of tone or sustain. If your covered by insurance, definitely take the guitar to the best repairperson in town to do that kind of work - or send it out to someone who does that kind of work if there's no one in town. Not everyone is skilled in repairing broken headstocks, so do a bit of research and find the best person for the job. Good luck!