WHAT WAS GOOD?
The actual 'mystery' of the film's title was quite good as Batwoman was secretly...
...three different characters! As well as this Batwoman's costume shared a similarity with the Bat-suit worn by Terry McGinnis in TV spin-off Batman Beyond (1999 - 2001), which was a nice little nod to where Batman got the idea for his modified suit from.
WHAT WAS BAD?
The film was quite dull.
Putting the mystery to one side the actual story itself was quite slow, lacked a lot of the depth of many other Bat-tales and featured dull villains like Penguin and Rupert Thorne. Batman: Mystery Of The Batwoman was nowhere near as good as any of the other animated Batman movies set within the DC Animated continuity and basically wasn't needed at all. Oh and David Ogden Stiers temporarily took over voicing duties on the Penguin for this movie and it just didn't work for fans who were familiar with Paul Williams' Penguin.