This spell is a huge energy waster. Pretty much instantly nukes my energy supplies, so I use it very sparingly.
However, it is also one of the most destructive spells in the game, especially to monks. The maelstrom itself is small, so usually you can interrupt at least 2 or so monk spells before they figure out what is going on, but that all depends ont he monk. The 25 damage a second is nice as well.
I always take Maelstrom to the UW/Fissure. It's a god send in the chaos plains, and against large groups of casters/bonds. It does'nt do a lot of damage but it's one of the best AoE interupts in the game.
Maelstrom is well worth the 25e, if it were any less it would be overpowered. The most useful thing about it is that it doesn't really require any points in water to be effective, the constant interrupt nature of it is far more valuable. If you are aleady preparing for a ether renewal nerf, elemental attunement makes the 25e a much easier proposition to swallow.
It's a constant on my sorrows furnace skillbar, thats for sure.
Once in a game one Elementalist casted 1)malestorm 2)speed meteor shower on the enemy healing ball - it got pretty ugly - 2 spells defeated 3 monks alone.