Playing around with the report-designer 1.0.1 I found some real strange
behaviour.  So it is not really usable.
1. I create a report and want to summerize some currency-fields. So I create a
new  function with will do this - well, works fine. 
In report details there are two fields with represents currency-values. I
formated them using the proberty "Format" in report-designer (see "_Bild1").
Trying the Report at this stage, the fields will be shown as "x.xxx,xx", so two
digits after decimal-point, but without the currency-sign. OK, that may be OK.
Then I create the function (see "_Bild4") and add a formated textfielt to the
report. I assign the same "Format" (see "_Bild2"), but the result is very
strange...  see "_Bild3".

2. Playing around with datefields you will found even strange behaviours. 
Insert a Datefield using "Insert - Date" - the Format is correct in desgn-view,
but without any format in report-view (the internal number is shown correct ;-)
). OK, you can reformat the field in Writer itself - this is a workaround.
Creating own function with datevalues I will get no values or even the internal
numbers (see pictures "_Bild5", "_Bild6", "_bild7".