Ok just got back from Quetta ! 
Let me tell u what I did.
I had to appear in an Interview at BUITEMS Quetta. I chose Sada Bahar Coach Service as its the best in my opinion. By paying 1400Rs ticket for 2 by 1 i.e. in each line there are 3 seats instead of traditional 4 seats hence giving you lot of space to relax and sleep as its 12 hours journey from Sohrab Goth Karachi to Sada Bahar Bus Terminal, Saryab Road, Quetta.
There are 2 timings that bus leave from Karachi to Quetta and Quetta to Karachi that are:
7:00 AM Morning (Booking time is 6:00AM)
7:00 PM Evening (Booking time is 6:00PM)
I chose 7:00PM Friday evening time as I had to appear at interview by 9:00AM Saturday Morning.
Bus ride takes atleast 12 hours. As I was in hurry and for those bachelor kind of people Sada Bahar Bus Terminal Quetta has a hotel as well, we were 2 friends and we opted for 2 Bed Room which cost us Rs.700/= for 24 Hours. REMEMBER checkout time is 2:00PM.
Rooms are fine, as per my policy if you want to see someone home or hotel about how they take care of it , see the bathrooms :D.
After getting ready we hired a taxi from Bus Terminal to BUITEMS Takatu Campus and it cost us Rs.400/=.
After interview we travelled from Campus to Quetta city through local bus whose fare was Rs.15/= per person. From city we hired a Rickshaw to Bus Terminal (back to hotel) which cost us Rs. 100/=.
In Quetta usually Saryab Road is troubled one as mentioned by fellow member here already. But Sada Bahar Terminal is secured by private gaurds and we felt safe there for sometime.
We had our booking that evening for Karachi in same fare for 2 by 1 seat bus. And it took 12 hours to reach Sohrab Goth Karachi.
The End.
Important Point:
Always take your lunch or dinner with you when leaving for Quetta to Karachi as the restaurent where the bus stops has worst of foods you will even taste.
While coming back always be prepared as they from Quetta to Khuzdar there is no stop i.e. 7 Hours of ride which means if in evening u leave Quetta at 7:00PM u will reach restaurent/stop at around 1:00AM. 