I believe I would take it.
To have the option to live countless lifetimes? This may be a cheat, but I'm sure that in the future at some point there would be a "cure" or a way to reverse it, so if I would choose to stop being immortal, I probably could, but again, this may be cheating the purpose of this question.
I think I would choose to take it, I am fascinated by what the future holds, the future that is far after my time on this planet. I think that many of the questions about the past could and will be answered in the future, to know them and to also live in the future, whatever it will be if it will be aliens, a new planet, whatever, is something that I would choose to take this opportunity.
I'm sure this would come with some (or much) torture as well since I would be outliving all of the people I care about (though if I am immortal, why can't others be also?). I'm sure there are other negative points about this, there must be as being immortal means also living through so much.
You also have the opportunity to do a lot of good that others, non-immortals can't do (since they would probably die).
So, I think I would take the opportunity.
