A somewhat embarassing teaching.  My impression of Mataji is that she naive rather than a fake.  Sahaja Yogis claim that all that is required to achieve self realisation (or at least this is what they said when I visited one of their centres some years back) is you put your hand over various chakras and say "Mother, forgive me" a number of times.  This activates the Kundalini.  At the end of all that you should feel a cool breeze over your crown chakra.  If you feel that, it means you're now an Enlightened Being.  Yes, well....

I do not know whether Mataji herself goes along with her devotees claim that she is the avatar of this age.  She comes across as a compassionate individual who may have attained some level of higher consciousness.
(note: all of this was written before I found out some of the sordid details of the movement from an ex-disciple!!!)

An ex-disciple replies to what I have written here - worth reading!

But consider the following behaviour by Mataji's disciples and come to your own conclusions:

Sahaja Yogis Threaten Violence

Reality Test for Sahaja Yogis

and here is a story that takes the cake:

Shri Mataji's Bid for the Nobel Peace Prize

Sahaja Yoga - Union with the Supreme - an essay by Chandra R Talware, basic overview.  Nice animated gif.

Sahaja Yoga: - Facts and the Truth - a very well presented critique

Is Sahaja Yoga Good or Bad? and a mailing list about Sri Mataji's Sahaja Yoga

Gurus page