Given the quality of Black Mesa, I wouldn't be surprised if Valve ends up with some sort of formal relationship with the developers (either hires them all, or adopts it as their own product to sell and gives a cut to the devs, or port to consoles, or release the Xen levels in conjunction with Episode 3 or something).
I've just finished it now. There are a few niggles with it (as experienced by myself and others on the official forums):
- The marines are too strong. They react far too fast - if your leg just barely peaks out from some scenery and they're 200 feet away, they'll start blasting at you full-bore. It's even worse when you're up close. The shotgun guys in this respect can be a bit easier since they pause to reload, but they do a lot of damage in a hurry. They're also very accurate from long range, more accurate than they should be. There are a couple of very difficulty battles as a result of this. Vortigaunts don't have a long enough charge up for their attack; Houndeyes are also quite quick to let-loose but given they were almost entirely non-threatening in HL1 this is a good change.
- Some levels are cut down quite a lot, particularly On A Rail (which I always hated anyway - new version is much more fun) and Surface Tension, which is a pity because the original ST was very fun, however the OP marines make this less enjoyable. Some of the later levels are verbatim HL1 with very few changes, if any.
- MP5 machine gun is too inaccurate and the clipsize is too small to actually kill marines with it, so almost never use it. This leaves you relying on the magnum and crossbow a lot. Also the glock is surprisingly powerful, I was using it right up till the end, mainly because the MP5 was so useless.
- A few crashes, particularly during Surface Tension, and very weird FPS issues with the Garg battle before Forget About Freeman.
- Assassin's way way weaker than in HL1, but they were quite frustrating there so maybe not a bad thing.
Also I found in BM that there were some weapons I pretty much never used, whereas I did use them in HL1 itself: snarks (marines kill them too quick), trip mines (no real opportunities). GLUON gun seems to chew through the ammo just a little fast, but very powerful.
None.