That's a tough question to ask me. I guess it comes down to, I'd press it, but I'd feel strongly remorseful afterward. I have my reasons my pressing it, seeing the world for a **** long time, living in the same place I've loved for centuries, but it's also mixed in with why I wouldn't want to live forever. Sure, I might not have so much things to do in this lifetime that I'd like to do one day, but when that certain time comes, the time when you realize, "What else do I have left to live for?", you don't really have much choice.
Relate life to what it is now, under the assumption nothing bad happens to you. Your life, RIGHT NOW, could be considered forever, 'cause it's gonna be a hell of a long time before it's up (again, under that assumption), you'll make the same choices, and keep relying on the present, therefore resulting in error and wishing you'd press the button. But when you do that, you could say you will eventually get bored of immortality, was it?
I think everyone has a time, and the choices you make influence your next choice whatever the hell it may be. But I'll be extremely selfish (wrong word in this context

) and press that red button.
PS: I don't think it's worth living for eternity so I sorta did this post on the button extending your life, but you can see it conflicts with the idea of immortality a bit more often than I'd like.