Act 3 MP0 is widely considered to be the most efficient area but you spoil the fun if you only ever farm the few zones there.
Farming higher MP difficulties are perfectly viable now.
I've farmed MP4+ almost exclusively since 1.0.7 and still get plenty of XP and legendary drops.
I've been farming Act 3 on MP0 to MP3 for many many hours. Must be at least 50 - 60 hours, switching between a couple of classes and I've found exactly nothing! I haven't found anything in the last 100 hours or so of game play. Pretty much given up on the game. The only thing you can do after beating to inferno on a few classes is grind for loot. But finding anything is damn near impossible, at least for me anyway.
I keep reading post after post with people saying "farm this area, legendary drops every 10 minutes!!!" or something similar. So, like an idiot, I keep playing and waiting for that lucky drop and still, I get absolutley nothing.
Actually, I did get 1 level 63 legendary crossbow yesterday that had a massive dps of almost 400! Took about 5 seconds to turn it into brimstone. I mean wtf a level 63 legendary with 360 dps?
One thing i've been trying to find is a decent wand. I can never get even a yellow one to drop. I finally got one to drop this morning, ilvl 63 - dps 271. Again wtf?