I've been using agaraMILK for ~2 months. My calcium levels have come up a bit (typically ran low-normal, now mid 400's). My coralline has taken off! Now, this was an established system that moved to a new home four months ago, so it may be that all of the details are finally stabilizing, but I've really have had some tremendous coralline spread. It covers essentially almost all rock surfaces. It's now on glass and filters.
My inverts seem to be happy also.
Like most things in this hobby, it's difficult to say whether or not one particular additive has a specific effect, but this seems to help. It is easy to use and seems to make sense.
I started using it based on the good LFS's rec in my area and reading some reviews on various boards.
Good luck,
Jaret