I haven't played Shadowgate 64, but I have played Shadowgate on the NES, so I guess that's better than nothing. From what I remember about Shadowgate was it wasn't bad for a point-and-click game, but after I broke my guy's legs for the third time in 10 minutes I never really picked it up again. In other words it was rather frustrating and didn't really have any appeal. I'm guessing that's what Shadowgate 64 would be like.
From the reviews I can find online it seems to be a pretty poor first-person adventure. It probably isn't really worth playing, but you can always just give it a try anyway and see for yourself.