In my mid 50's, my enchanter collected up some negative magic resist gear for charming. The first time I tried it was in Howling Stones. I lost everything by botching a pull and feeding my pet two rooms and a hallway worth of harm touchies.
Sour-graping my loss, I've held off on getting more but I honestly wanted to know how effective it is. So in the name of science, I put together a -50 MR suit using the following:
Next, I went looking for some blue-to-60 mobs to test on. There were a few things I wanted; minimal risk of killing me or forcing me to gate, to be able to get my items back, someplace quiet where other players wouldn't unintentionally interfere, and mobs that would be unbuffed with both hands free to hold items.
There's a tunnel in the back of the giant fort in Frontier Mountains with two level 50 warrior giants; Eldak Howlingbear and Mentrax Mountainbone. They are non-aggro, non-social to each other (or anything else), and drop nothing.
WARNING: Mentrax is a quest npc for the warrior epic and his corpse will not return items when killed. I almost learned this the hard way but thankfully tested getting a torch back from both giants before I started.
The first test was the control. Level 60 enchanter, 255 charisma at all times, Tashanian (-39 magic resist) on the pet at all times. Since Tashinian lasts 13 minutes at 60 and Allure lasts 19 minutes I decided to break charm when Tash ended and recharm. I would not count these as early breaks. I ran this test for one hour, counting down. All I did was sit there with Eldak charmed ready to root, tash, and re-Allure. I had early breaks at:
59:28
48:16
42:58
31:52
28:39
22:03
20:26
1:37
With a deliberate break to re-tash at 7:27.
For phase two, I used the same spawn of Eldak and gave him the seven -MR items and kept him charmed for another hour. I had early breaks at:
46:02
25:49
17:29
With deliberate breaks to re-tash at 48:03, 31:56, and 4:24.
I have to concede that two hours of testing is REALLY SMALL to draw definitive conclusions but it did appear to be effective.