I for one simply cannot imagine "2001" without the classical music score. And it's not as if György Ligeti's choir music wasn't "contemporary" and still remains somewhat futuristic (or at least timeless). In conclusion, I'm with Kubrick here. It was the right choice to keep the "Ersatz score". The other music isn't entirely bad, but at the same time somewhat painful to listen to in its attempt to imitate the Zarathustra theme without plagiarizing it. It's probably highly unfair to say it, but my (highly subjective) impression was:
"Meh."