Hi,
I am unsure of Mennonite record cards on Ancestry. (There is such a thing??) Now I have to go look. :)
But as far as Grandma is concerned, I believe it is well worth it's cost for me. I have found many Mennonite names and ancestors of mine. Almost all of my ancestors are from Canada, having moved to Canada from Russia. There is a wealth of information on there! Having said that, they do quote their sources where available, but like most sources, unless you have the actual records, there is always the chance that they were said, heard, or written wrong. Most of the sources are from the Mennonite church records, so I am not sure how those could be mistakes, but...*shrug*...you never know. But I do love using it for, if nothing else, finding names and dates I can look further into at a later time.
Hope this helps!