I have heard of it a while back but i've never really been that interested as i've played all the Half-Life games already, i'm guessing all this is is just a graphical overhaul or do they change anything more significant?
It's not just a graphics overhaul. It's been completely remade from the ground up. Nothing from the original HL has been used in BMS, every model, texture, map, voice over, sound effect, everything has been remade.
Also they've apparently tweaked some of the levels so they flow better, have extra areas, etc.