Generally rather be caught dead than link to lesswrong; however, this post on the "Lifespan dilemma" is sort of interesting if you can grit your teeth and deal with its spectacularly awful prose. I haven't read the post esp. carefully but it seems like there are two aspects to the idea: (1) a lot of math that seems basically irrelevant, (2) the clever idea of reformulating Parfit's paradox in terms of years of life rather than numbers of people. Personally I don't think Parfit's paradox changes when you formulate it in terms of lifespans than numbers of people -- the correct unit is clearly people-years, rather than people or years -- but I guess this might not be widely accepted.
I'm an average utilitarian* with respect to Parfit's paradox, despite the somewhat compelling counter-argument that this should lead one to prefer having thousands of people mildly tortured to having one waterboarded. I would like to reconcile this with a system where, like Parfit, you have a floor of utility for a life to be "worth living," and (unlike him) adopt average utilitarianism for people above the floor and stoatal utilitarianism for those beneath it. There seems to me no a priori reason why one shouldn't use different metrics for lives that are worth living and lives that are not, and this metric matches my intuitions best, even if it seems inelegant.
And I think I'm also an average utilitarian* with respect to timescales, which sort of explains why I hold the views on longevity (i.e., that there's no intrinsic merit to it) that I do. I should emphasise that these actually are my intuitions on the matter; I'm not trying to be contrarian, I just actually disagree that longer lives are better. The appropriate measure is utility per second averaged over one's life; a longer life is better to the extent that one can expect this to increase -- e.g., as an adolescent you'll probably increase your average happiness by staying alive a little longer; this also accounts for situations when you're 65, dying, and want to be kept alive long enough to e.g. check out an impending grandchild before you kick the bucket. One could also argue that as you grow older you've read and experienced more and therefore find life richer; therefore there's a steady upward trend to one's utility per second. I don't think this is true beyond a point, though, and there's a tradeoff here which Wordsworth's one good poem is very good on: one gets contemptible, stiff, and dull with age. Some of this has to do with the physiological processes that cause aging, but I feel like some of it also has to do with just having been around for a long time.
* I'm not a true utilitarian. However, like most people I'm a utilitarian most of the time, so it's meaningful to ask whether I count average or stoatal utility.