I've never rated a show "perfect" before, but that's exactly what this is.
The script is so smooth and the jumps between eras is hardly even noticable: I'm just as enthralled by Erin's grandfather as she is.
There are scenes where there's just silence, and these are so subtle and well-timed. Not to mention well-actred.
It's the mixture of all these things that makes the show so perfect though. I can't find one fault with any of it.
You can imagine that the show will end with the grandfather's death in the current day, but even knowing that doesn't annoy me. Only one episode left and I'll be sad to see it go. This is one of those shows you'll be able to enjoy again and again.