I’ve actually been playing through Gold and Crystal on VC lately, so I have some thoughts.
First - visually, they’re a little poor. There should have been a better scaling mode to display the games with a decent size but a sharp resolution. Other systems can display them just fine - the 3DS line is stuck with either a slightly blurred full size view that looks like something has been smeared across the screen, or a tiny, tiny original resolution view with an unnecessary border.
Beyond that ultimately minor gripe, they’re phenomenal. It feels great to play my first video games again, they hold up exceptionally well, and like has been mentioned, Crystal allows us to encounter Celebi in the US finally. Crystal additionally has a Battle Tower that laid the foundations for two decades of battle facilities and is quite fun on its own. Plus, we can easily and seamlessly trade with other devices with VC games. All around I’m quite happy.
I think my favorite part is probably just getting to play through Tin Tower again in its original form, or to hear the "Road to Cerulean City: Leaving Mt. Moon" theme once I’ve made it to Kanto… nothing is more nostalgic.
Honestly, I deeply love HGSS, but I return to GSC VC much more often. I hope they’re made available soon for the Switch with the next NSO system, assuming rumors are true that GB/GBC games are coming eventually.