I'm not sure about flex vs. non-flex, but I use them for everything. I have my farm mortgage through them, revolving lines for operating and equipment loans. Nothing I have has a prepayment penalty of any kind. They will also reamortize my mortgage and lower my payment with no fee on my whim if I make a big lump sum principal payment. I have nothing bad to say about them. You do have to buy a little stock to borrow from them, but I felt that was miniscule.
