The concluding epigraph from Homer's Odyssey in Book 3 sums up this book perfectly: "…and so she too rejoiced, her gaze upon her husband, her white arms round him pressed as though forever." It's a bittersweet ending because we know that this long-anticipated reunion won't last. Henry will disappear again and then who knows? Next time Clare might not be there to welcome him. Of course that is only the case for Clare, since Henry can always visit her in the past. It doesn't seem fair.
Then again, the question remains: what's better or worse? Traveling back to the same moments, warming up old joys while also reopening old wounds, or knowing that there's an end to everything. Except for love, of course. Love never ends! Plus, physicists explain time in the universe as warped space, meaning eventually time just circles back on it itself. So, sounds like the end is just the beginning…