It's by far the most incredible mod ever made. No offense to the ESF folks here. These guys have remade everything from scratch. Replicating nearly every map from the original game flawlessly, and then updating it's appearance and gameplay to both enhance it's graphics for modern day, and to capitalize on and fix some of the flaws.
They've even (as you can tell in the trailer) thrown out all the audio from the original game and have made custom sounds, dialogue and everything. And it's not all copy-paste either. They've added some things as well.
2009 seems to be a year of anticipation, with the Dragonball movie to come out in the Spring, and this mod to be released sometime in the year.
I'm thinking that when I educate my children one day in great classic video games, I'll have them play BM:S instead of the original game. I'm getting the feeling there will be little point to play the original Half Life with this mod in existence.
On another note, I really hope the BMS makers allow our local npc guru at facepunchstudios, Silverlan, to use their models to put Black Mesa Source enemies in Garry'sMod. That would rule so...SO much...