To answer your question Mandy, a lot (I would say majority) of people put the weight back on. Then again, I think they say that about all diets, so take that as you will. I know a few people who've gone on shakes. Of them, four of us (including me!) did it for a few weeks to a month and lost interest. Its hard to stick to (pretty low cal) and personally I got bored with the lack of options.
Of the remaining three, one put it all back on and them some, one put some of it back on and one sucessfully maintained.
This is all ancedotal of course, but I think you'll find it an ok representation of the overall demographic.
For what its worth, my friend whos maintained it lost about 40kgs all up (I never asked ehr starting weight, but shes 60kgs and she did say once that she was triple digits) and she was on and off the shakes during her weightloss period. She also got really into (and has kept up) a gym membership.
Personally I think shakes aren't all they're cracked up to be, but given the right person and circumstances, they can give a person a nice boost. I would only stay on them for a short time - like I said, they're pretty low calorie. Also, not something I've checked in to myself so take with a grain of salt, but I've heard the Aldi shakes are exactly the same as Tony Ferguson and they are a dollar cheaper each. On the flip side they have even less selection.
HW: 84kgs (2011) got down to 67kgs (2012)
SW: 74kgs - (2013) post baby!
CW: 68kgs (Updated 6 May 2014)
GW: 62-64kgs (size 10)