I don't have much experience with all the green supplements offered.

With all that chlorophyll you'd get some magnesium out of them.

I don't think they are harmful, if you choose a company with a GMP guidelines...but with all the business with the sprouts coming out now...being contaminated...I wonder if those pathogens could be spread in a supplement too.

The sprout thing I've followed has included France, Idaho, and Egypt as sources of dry seed...to be used in the sprouting.
Once wet, the organisms wake up and infect you.

I'd be careful with blue green algae right now too, because new studies have found a substance in them that is toxic to the brain:

http://neurotalk.psychcentral.com/thread152771.html

I tend to not use the "green's" mixes, since they tend to be expensive and I need that money for other things I feel are more important for me. So I cannot say from personal experience if they are effective or safe (these days).