It depends on how would you body evolve (for example, would you still get white hair at 50 years and almost not be able to move at 90?) - provided you're keeping some energy to move, talk, interact, and understand what's around you, then it would probably be amazing to see how the general society evolves (someone from 500 years ago would barely believe what they would be seeing today if they were still alive, and that'd probably be the same case for someone of our era who would get to see year 2517).
Spectating the general evolution of humanity, as well as the one of each country, would just be a chance people would dream of, because not only would you accumulate a lot of knowledge, but you'd also have the opportunity to say "I was here" -- not a lot of people today can say they've lived during both World Wars, most of us smogoners haven't even lived during the era in which there were two Germanies and USSR was a thing. Seeing the world changing and evolving would be amazing imo, especially when said evolutions usually take some decades to happen.
Obviously though, if you're the only person to live forever, this means you're not only going to be alive in 500 or 1000 years, but in a million years too -- by then, humanity would probably have disappeared since a long time and you'd be the only one left, so there wouldn't be a lot of things to do despite swimming in the universe until you find another inhabited world. But at least, you'd
know how it all ended, what happened before, etc... That'd be super cool.
That is, provided you're not getting weaker with the years. Currently a 80-/90-year-old human can't do much and barely communicates, so it'd be way worse when you reach the age of 200, 300, 500, 1000, etc... years. Being there to contemplate and remember stuff is interesting, but only if you're able to remember they happened and aren't wondering what you ate yesterday or what the names of your children are.
So if it's confirmed that such a choice would make you become a super weak grandpa with the time, I'd rather die right now. What happens after one's death could be interesting as well -- it's so cool that nobody has ever come back from it to tell us about it
(or maybe does nothing happen). Otherwise if it's confirmed that you're staying "young" or at least, not becoming a vegetable, and even if it's not confirmed and people do not know what would happen.... well, I'd still try it!