I have used Himalaya Travels twice to go to Darjeeling. They were urgent trips so I had to take the sleeper bus by Himalaya travels.
The ticketing process was smooth and easy and I faced no issues what so over in that journey. They have such a huge network that they have their own bus stops in Darjeeling and even on the way. The buses ran mostly on time.
The seats were clean and the light and charging points worked. The bus attendants were very courteous and helpful. I had heard about the the roads being really good, everything being orderly and neat. Eager to go and see it I took the Himalaya travels. Give it a try if you are going to Darjeeling by a bus.