Many spells here do and don't work.
Whether or not a spell works depends on a few things.
Personally I suggest you use common sense when looking for a spell; for instance if the spell is claiming something crazy like, "To Become a Mermaid" that should be your first hint that it's bogus.
Also if you truly believe in the spell, and really understand it, then there is a much higher chance of it working.
This being why it is suggested you study spellcraft basics before practicing (casting spells), because once you are a bit more knowledgeable in the field, then choosing good spells or even writing your own, will be much easier.