Are there any good reasons to think that life has intrinsic rather than instrumental value?

First, let me offer a gloss on your question: By 'life' here I take take you to mean something like an individual person's being alive or continuing to live, as opposed to all of human life, or biological life in general.

I must admit that I cannot think of any compelling reasons to believe that life has intrinsic value, that is, value in its own right or for its own sake. When we reflect on our reasons for wanting to continue to live, we might say 'it's great to be alive!' or 'ain't life grand?' But appearances notwithstanding, such remarks don't seem to amount to saying that merely being alive, apart from the quality or worth of that life, is valuable for its own sake. Rather, we seem to have in mind that there's something about life that is great and grand, apart from simply being alive. If we try to imagine just being alive, is that a good state to be in? As soon as we are tempted to say 'yes,' we are likely to start referring to facts that would make life good but not for its own sake: the prospect of pleasure, pursuing goals, etc.

That said, many of us have an intense fear of death, and will go to almost any length to continue our lives. No doubt some of that is due to natural instinct. But even there, our clinging to life doesn't seem to rest on life's being intrinsically good. If it makes sense at all to fear death, it makes sense only because we see prospects of good, however dim, in continuing to live. Life beckons us because of how it can go, not because it is a valuable condition or state in its own right.