From what I remember the AI was the same as well as level design.
See: Crysis.
Awesome graphics. Crap story.
I’d say it depends:
If you want to play Black Mesa to see if it’s a respectable remake of half-life(source), go download.
On the other hand if your just playing it to complete the plot in the half-life saga, then u can make BM your first experience. It’s either that or your exceedingly cheap…
I go with the second one… namely being a cheapskate 
Just watch Freemans mind and have a laugh at the same time 
hasnt everyone here played threw HL at least twice?
No, there are a lot of people here who were introduced to the Half Life series by Half Life 2 and never actually played through the original story. Some people are specifically NOT playing Half Life so they can do Black Mesa as a completely fresh game (which will make for an interesting perspective) but many holdouts have changed their mind when they realize the goldsource mods they would get access to with the original game.
Personally I’ve been completely through half life about 4 times, once on hard. It’s hard to be completely accurate because there have been a few times I’ve played through and got distracted for a couple weeks in the middle so I started back over at the beginning. I think I’ve only played through Half Life 2 about 2 times total and have yet to play it on hard.
The main reason to download Half-Life: Source is this: It takes the old textures, models, etc and adds the new source engine physics. For example, in the original Half-Life, when you shot someone they would shake in place and then fall backwards. Or if it was a soldier, they would slump and fall to the ground and all of them died similarly. However, with the source version, they all die differently depending on where they are shot and the interaction with the environment. The water, particles, and explosions take their queue from the source engine as well. Everything reacts different because of the source physics as opposed to the old version. I have both, but mainly play the source version. I keep the older one around for nostalgia. However, I purchased Half-Life Source when it was on sale for $5. Now, as of May 9th 2010, it is $9.99.
I think you should just wait for BM for a little longer
Playing HL:S will ruin BM, in my case
NEVER EVER DOWNLOAD HL SOURCE, just get normal hl.
I have played hl1 b4 but I think it was boring, so wait for BMS its the best option!
If I were you I’d at least play through Half Life. Source or not, you should have the experience before playing BM.
Yes 
And HL Source is slightly better and less buggy than HL Vanilla, so use that one.