well, a gaming laptop is never going to be the best value for money deal. Yes they are heavy and barely upgradeable, but people buy them so that they can move their system around, or if they don't have space for a desktop. I need to move around often, and I also enjoy having all my peripherals packed into a single relatively compact device with only one cable sitting around. You can't really watch a 3D bluray movie in bed with a desktop PC, or have LAN parties at friends' houses. you can with the G750. Also, it has a MUCH lighter impact on the electricity bill, which people forget VERY often.
A LOT of people don't understand that and only see the downsides, but that's just how it is.

If having a laptop or a desktop is really the same for you, then for 1200 you could have a gaming rig with the specs you want (a GTX 760, for example, would be 60% faster than the 770M in the G750JX) assembled by a professional and handed over to you "ready for use". A lot of tech shops offer that service, and that is probably why your colleagues at work think you overspent with a gaming laptop. It's really a matter of how you plan to use it.