If you decide to travel by bus to Goa from Mumbai, bus by Goa govt "Kadamba" is one of the best options. Its cheap (I mean the rates do not sore as in case of some private operators); and moreover, they are reliable and services are also good.
1. There is 1 bus daily to and from Mumbai, leaving Mumbai (Vashi) @ 8pm and reaching by 5am at Panjim. Bus from Panjim leaves @ 6pm arriving Vashi @ by 6. Ticket should cost approximately 700. You can buy tickets online, as well.
2. Its not a multi axel Volvo thus seats are not that comfortable; however, buses are in good condition.
3. It goes via old Goa highway, thus, be ready for lots of twists and turns the whole night and if you have sickness due to twists and turns, be prepared with required medicines in advance. They do provide with sickness backs, if you need.
4. Its a govt bus so they do adhere to timings. If you have to catch your office once you are back in Mumbai, you can trust them to reach on time.
5. Toilets at halts on the way were reasonably clean. I am not sure about the food in the restaurants though. It however appeared expensive.
6. Bus staff too was courteous and helpful.
Remember that it is NOT allowed to carry liquor from Goa to Mumbai (even if you have buy permit from the liquor shop as they make it for Daman, not for Maharashtra). There is heavy police checking once you enter MH and if caught, they will confiscate the bottles. Its however not a crime though if you have 1-2 bottles and you won't be booked but better not to carry at all rather than giving it to cops. If you travel by govt buses, risk multiplies. My bus was made stop 3 times by MH cops that night and whole bus was thoroughly checked by cops.