I used Messenger HG once a while back. AFAI can remember, I noticed no ill-effects and the outcome was just fine. Of course, I had no control group and collected no real data on the grow whatsoever (data which would be virtually meaningless without control data anyways).
Just so others here understand just what Harpin protein is: harpin proteins have been identified as being produced by a number of bacterial plant pests. Apparently, there is a protein harpin EA and another harpin alpha beta protein. The later is supposed to be more effective. The idea here is that these proteins bind to receptors in the plants immune system, and this indicates to the plant that it is under attack. This in turn increases defenses and stress response, which results in increased vigor and production over-all as there is no actual infection. The protein(s) are merely tricking the plant into thinking it is being attacked; much like how our vaccines work by introducing killed or inactive/attentuated forms of pathogens into our bodies to produce a very similar immune response. In fact, it is a lot like what would be referred to as a subunit / conjugate vaccine, which uses just a portion of the virus or pathogen, such as specific pathogen proteins as antigens.