In another life I was a Wizard. Wizards get all kinds of rain spells. There are some tricks to properly using a rain spell.
For one, if you are concerned about mana, this isn't a spell for you. Typically, you will want to use a rain spell if you are wanting to max your DPS potential by:
1. casting your rain spell
2. while your rain spell is firing its 3 waves you use your regular DD spell
In essense you have the potential to hit a mob 4 times in the time that you would normally only be able to cast twice.
Now, there is some issues with rain spells. Parses have shown that rain spells have a hardcoded chance for resists (20%ish i think) aside from the regular resist check, which kind of sucks.....
Hope this information is useful.