Before I give you my opinion on this, ket me give you a bit of background.
I last played HL1 in 2004, just before getting HL2. Didn’t play any og the episodes yet. I planned to do another complete series run-through and episode 3 was out (haha, yeah, still waiting for that) and when it was clear that ep3 was never coming, I had already heard about BM and I decided to wait for that with my HL runthrough.
That was back in 2009 and now, after forcing myself o keep away from these games for more than 4 years, BM is finally out.
So what I did was to play Black Mesa, than start up HL Source (no need to cheat there because you can start a new game at any chapter) and go straight to Xen there (shouldn’t take more than 3 hours or so on normal, so it can be done quickly and easily.
Now, I am in the middle of HL2 (which is just glorious as well).
Let me say that I was amazed how easily the transition from BM to HL Source went. Sure, there are a couple of differences in weapon handling and obviously the graphics suck (especially when coming right after the amazing spectacle that is Black Mesa) but since you have that immense contrast in Level and visual design it works pretty well actually.
I am sure, the Xen chapter of the BM team will be vast improvement and given my long abstinence, I’ll probably just o another complete run through in a year or so when Xen is out but for now, I thought it worked just fine and you have a sense of continuity.
So there, that’s the way I would recommend it to anyone who wants to charge on after lack Mesa and I can say that I am now in the fortunate position of still having the completely unknown and unspoiled content of episodes 1 and 2 to look forward to.
Now if you will excuse me, I have to get back and rescue a certain Dr. Vance from Nova Prospect
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EDIT:
Sorry friend, but I think you are mistaken on that one. I remember reading a test of HL1 in my favorite PC games magazine back then and there the graphics were judged adequate but not top notch, certainly not when compared to the graphical superstar of the time - Unreal. 