I just did MK After Hours this past Friday, (MLK weekend). It was good, I thought though that it was more crowded compared to other reviews that I have read.
My 3 hours consisted of heading over to Big Thunder, got there around 9:02 pm, had to wait about 15 minutes, I guess the regular crowds had to clear out, went a 2nd time right after. My biggest complaint about this ride was both times everyone had to walk up the regular queue line which involved walking around additional massive amounts of turnstiles for nothing. We headed over to Haunted Mansion, no wait. Went to do the Rapunzel Picture with the cube, but the line was a bit long so we hit up Peter Pan, 5 minute wait, then back to the Rapunzel picture, 4 minute wait this time. Line for 7 dwarves was really long, sign said 20 minutes, decided to hit up space mountain twice no wait, no turnstile hassle here, then astro orbiter, buzz light year. Met both pairs of princesses in fantasy land, probably a 5 minute wait for each side. Decided to go back to the Rapunzelle picture thing again. Then with just a few minutes left before midnight we hit up 7 Dwarves, we had to wait about 10 minutes.
Then back to the bus. I went to the Swan/Dolophin queue, but they combined Boardwalk, Yacht Club, and Beach Club together to 1 bus. The first bus was there when we came up to the line and had just started boarding. We were too deep in the line to get on this bus, about 10 minutes later another bus came, this time was able to get on, but barely was able to make it in. Remaining in the queue was about 1 bus load of people. Some people near us were investigating a lyft back and it was going to be $36. I am not sure how much they normally are.
So overall it was a great time, with the exception of the crazy turnstiles at big thunder and the 7 dwarves line, everything else we went to had generally no wait.
That said I think this past weekend was a very crowded time of the year the parks, so I think that the crowd size at after hours was also larger.