value is a key word and represents the value on the connection before applying the filter function. The returned value is the one that is used in the bound-to attribute. I found that the expression has to be a one-liner (technically), i.e. you have to start your expression with 'return' and can't do something like:
I haven't tested yet if these filters can also be applied to input arrays of objects that come from a service (e.g. for a repeater widget). If this is supported the array-based filters may be applicable/usable as well.