Superb. The best example of a slow, slice-of-life anime. It's an introspective manga that takes place over a massive number of years. there's no overarching plot, just the day-to-day happenings of a humanity in it's twilight. It makes you think and feel in abundance, and Alpha is a great protagonist to view all this through. Honestly of all the post-apocalypses in media this is probably the most beautiful.
Honestly I could probably go so far as to say it's quite a philosophical manga, but that might be overstating its intentions. I really do think this is mandatory reading for SoL fans, or introspective thinkers in general really.