I agree, it might not have aged in stellar fashion, but in 1999 it was a very good adventure. I also agree, that in a sea of mediocre 3rd party efforts Castlevania 64 was a legitimate AAA game from Konami on the system, and as such I was thankful. To be fair for Goemon Mystical Ninja was also a pretty darn good game from Konami for the system.
Goemon was a legit good title that I wish were available today. I liked it better than any other N64 title that wasn't made by Nintendo of Japan, and yes, that includes Rare's games.