Yes I have used it... and liked it. However, I have used more MightyPlant made by the same folks. It has a touch of Messenger in it, the rest is fertilizers. Frankly I use it so seldom that I really forget what's in it. I try for the most part to use compost, alfalfa tea, fish emulsion, well-rotted horse manure and such...
Messenger is great. However...be sure that what your plants are going to take up is healthy for them. I nearly lost my Greenhouse full of plants when the salinity was high and I didn't know it...yep...the Harpin Protein encouraged the plants to take up the salinity and...well...it was not a happy time!!!!
Messenger is harpin technology... makes the plants increase their defense system. Mighty Plant, by the same company makes a liquid foliar and root fertilizer with a bit of Messenger in it. Best of both worlds if you are using non-organic fertilizers.
I also have Mighty Plant which I use outdoors...it is good. I nearly lost my whole greenhouse when I had some coir in my mix reabsorb the salts (coir attracts and holds salts) and then I used Messenger. Yep...the plant guzzled the good stuff AND the salts...causing all kinds of problems. Had to repot about 4000 plants in a hurry!!!!
I grow Hoyas. In order to be Certified to ship out of Hawaii (by Hawaiian Ag - don't ask, it is complicated), we have to grow under controlled circumstances which need a greenhouse. I only grow the hoyas in it...everything else grows outdoors except for stuff which may not like all the rain we get!!!