You get up in the morning and take a taxi higher into the mountains to the reserve, getting out and walking further up the mountain's roped off trail.
The trail terminates into a small, flat observation area. And if you go early enough, you might wonder where the hell all the butterflies are. But on closer inspection of the tall trees around you, you'll notice most of what you thought were leaves and bark are really clumps and clumps of sleeping (and half-frozen?) butterflies.
Once the sun comes out, these masses of butterflies seem to wake up and stretch their wings, warming them up before descending the mountain in one never-ending flock, landing in a grassy clearing (about 50m back down the trail) to drink from a stream and frolic in the sun. Butterflies will be everywhere, swooping past you in incredible numbers or landing on you -- especially if you're wearing bright colors.
Cool experience, charming little town, and beautiful landscape. As things become more dire for these little guys, might be worth a visit.
I thought Angangueo would've been warm since it's a migratory winter destination, but it's super cold and colder than most of the climates the butterflies abandoned in the north. The place we stayed at had no hot water or fireplace. But a little bit of adversity, like seeing your own breath while trying to fall asleep, makes a travel experience a little richer upon recollection. :)