Praziquantel is a dewormer. It will not harm your biological filter, corals, etc. It is completely reef safe, and I have used it in my display when I suspected a fish had intestinal worms. However, it can kill feather dusters, and bristle worms, so keep that in mind.
As far as its effectiveness with flatworms, it does not kill red planaria flatworms or AEFW (in my experience). I have had flatworms live through 3 rounds of praziquantel, unaffected. I think AEFW are even tougher than planaria (since products like flatworm exit doesn't even work on AEFW).
Once again, just one experience, but that's my take on it.