Personal Opinion: Its a good place if visited from October last to February last. Other time it is not good due to sunny weather.
Accessibility: It is 30 Km from Dehradun and can be reached by Bus, Car or Bike. My personal favourite is Bike.
Local Sightseeing: Lal Tibba and Kempty Fall are the best attractions in Mussoorie other places are around 20-30 Km away. The best part is visiting the local market(Mall Road) in evening.
Hotels: There are many hotels but I will recommend Jain Dharmshala which will cost Rs 400 a night for 2 people.
It is very safe and beautiful place to visit but only in WINTERS.