With the HD pack this is basically a remaster. It looks noticeably better. Water is essenailly Halflfie 2's with rag-dolls and cast shadows carrying over. The lighting is improved and third party effects such as Nvidia's ambient occlusion can be forced for slightly better scene depth.
It's not all good though. I noticed a few bugs. A scientist floating in the air, damage noises when no damage is dealt, the entire planet creature section ruined by an unkillable Barney.
Oddly as well Soldiers no longer seem to emote, goading the player or showing communication, scripted radio segments no longer work either. This is probably the biggest issue with the game, it strips them of any character. And for that reason, the original still trumps it.
If you've played Halflife several times though, and don't mind issues at the expensive of upgraded visuals, it's a good deal on a Steam sale, or if you're waiting for Black Mesa to be finished. If it's your first time round though, stick with the original.