We used mortared in red engineering bricks .. it's worked very well for me. They are 900 mm high on the high outer sides which is ideal for me and a bust up back etc.
T'was fairly expensive , but should last me out by many years , It's part of the whole garden landscape exercise .. Done by a dam good builder who really enjoyed doing something different & interesting .
Due to each wall being laid on a foot thick x a foot wide concrete foundations , in the middle of each bed there is a natural drainage hole . I also got the builder to put weep holes in the outside walls at every mid point of the beds lowest course of h bricks .
All the divider walling is keyed in brick work which adds to the strength tremendously .
They are filled with a home made soil less growth medium which is the best stuff for growing plants I've ever come across .