All the pre-Xen levels are very well finished. If you just want the single player gameplay up to the point of the Lambda Complex, install Black Mesa now :-).
You can get a preview of the gorgeous Xen graphics in one of the rooms of the Questionable Ethics chapter.
Whenever I used to replay the original Half-Life (I think I've completed it 5 or 6 times), I would get as far as Lambda Complex, pretend that was where the game finished, and promptly stop playing.
I think plenty of us hated the Xen levels, and the BM:S team knows how challenging it will be to modernise. I'm looking forward to see what they do!
I personally enjoyed the Xen levels (except the fact that Nihilanth brought my PC to a near standstill when the game came out), but to each their own. I'd be interested to hear your complaints about Xen.
A common complaint involved the tiny little orbiting platforms that had to be jumped between. It turned an excellent FPS into a frustrating 3D platformer and interrupted the rhythm of the game.
You can get a preview of the gorgeous Xen graphics in one of the rooms of the Questionable Ethics chapter.