Here is a very good thread on the topic that will help educate you on how it works:
http://www.project1999.org/forums/sh...ht=memory+blur
The short answer? It's a bit of a mystery exactly how the success percentage works.
For the time being it will be more efficient to make your tank taunt twice then kick the mob when breaking mez, then forcing you to recast memory blur countless times.
Or you can get somebody to root the mob (preferably not yourself since it adds still more agro).
It can be useful. I use reoccurring amnesia on raids but the success rolls are... well they are random...