I built an Enderman farm following the tutorial put out by a youtuber. Here is his video:
https://www.youtube.com/watch?v=uRSY13xhI9s

My center block is 150 blocks away from the main end island, and my walkway is down at y=1. I have three spawn platforms, using snow blocks. The first spawning platform is about y= 44, and the third platform is about y=52 (give or take a block or two).

I built it recently under version 1.2.9, and it worked pretty well. Enderman spawned pretty quickly and fell down the chute, to a mob cap of about 9 or 10. As I killed them, more would fall down.

Now, since 1.2.10, none spawn and fall down while I'm on my user walkway. However, when I pillared up to verify my endermites were still alive, they began to spawn and fall down. It appears that I need to be closer to the spawning platforms now for endermen to spawn and fall into the trap. I can't be certain if they are spawning and just standing idle until I get close, or if they aren't spawning until I get close. They will spawn and fall into the trap to a cap of about 6 while I watch. Then I have to go down, kill them, and come back up before getting any more. This behavior is the same whether I'm at the 4 chunk, 6 chunk, or 10 chunk tick radius (I don't believe I've tried the 8).

I didn't see any other endermen farms when searching. Sorry if this is a duplicate.