Is this a bare bottomed tank, or do you already have substrate in it? From what I have read, you don't want to bury established substrate.
As for it being messy.... I have never added it to a tank already filled with water. Don't know if the other people who posted have done this.
I know if you put it in a tank, and then fill it up, it isn't messy.
But when I did one of my tanks, for some reason, I went and stirred it up (never did this before..) after partially filling the tank and the water became extremely cloudy.
I also noticed an ammonia spike in one tank when it was set up, so you may want to watch for that.