Narrated Asma bint Yazeed (ra), the Prophet (saw) said:
On the Day of Judgement all men will be gathered on a vast plain, and the herald of the Lord will call out: "Who are those who foresook their beds at night to cry unto their Lord in fear and hope?"[32:16]
Those who used to pray tahajjud will stand up at this announcement, and their number will not be large. They will then be allowed to enter paradise without being brought to account. The rest of mankind will then be commanded to report themselves for the reckoning.
[Related by al-Bayhaqi)
The Prophet (saw) is also reported to have said that the most rewarding prayer after the obligatory prayers, is tahajjud. Therefore we should insha'Allah, make effort to pray as much nafl and sunnah prayers as we can, (especially the tahajjud prayers in the glorious month of Ramadan) because of the immense blessings in them.