It's great if you are planning on selling the car tomorrow, not so great if you want to keep it for more than a few days.
For sealing headgaskets, see above. It will likely increase the pressure in the unblocked channels (the ones that are leaking into the cylinders at the moment), causing it to fail even faster.
My dad just changed the coolant on his Rover 416; the coolant was a bright yellow/green colour, caused by the use of similar stuff by the previous owner. When we took the rad out the top and bottom 1/3 were completed rotted through (but not leaking). After finding that it was pretty obvious why it was getting warm when sat in traffic.