Can you believe the marijuana plant is the latest rage in proteins for bodybuilding? Well, that's a slight exagerration, but it is true that hemp protein is a growing segment of the market and there are many good reason for it.
First of all, the hemp used to make hemp protein has a very low THC content and THC, of course, is the psychoactive phytochemical that does what marijuana is famous for. And, for the unitiated, the hemp plant has been widely used for centuries for food, clothing and various other commerical materials and processes.
Hemp's popularity with muscle building was an outgrowth of the recent search for healthy, clean proteins. As I've mentioned elsewhere on the site, the old days of raw egg shakes and sucking down a couple of pounds of grilled meat are long gone for everyone except NFL lineman. Unless you are exercising like a professional athlete, all the arachidonic acid and saturated fat in these proteins is going to take you out of commission relatively quickly.
This is where a plant-based protein such as hemp comes in. Each 30 gram serving is packed with heart-healthy magnesium (180 mg or 45% RDA), potassium (330 mg or 10% RDA) and omega-3's (630 mg or 39% RDA). So instead of taking out your artery (and erections) with a bunch of artery-clogging inflammatory and plaque-building foods, hemp will do quite the opposite.
In fact, hemp will also likely help with the "pump" that weight lifters crave. Anything that helps with blood flow will likely help in this way and hemp is packed with both L-Arginine (1.6 grams) and L-Glutamic Acid (2.2 grams). These will boost Nitric Oxide, lower blood pressure and other good things for your health and muscle. 
What about amino acid balance? Unfortunately, hemp scores has a rather low PCAAS score, which is the metric used now to measure a protein's digestability and general ability to be used by the body. Eggs, casein (the primary milk protein) and, interestingly enough, soy all have the highest PCAAS score possible: 1.0. Hemp's PCAAS rating is significantly lower: 0.46.  The reason for this is that it has proportionally low values for about five of the eseential amino acids. What this means is that you have to eat substantially more hemp protein in order for the amino acids to actually be able to incorporated into your muscle tissues. In other words, some of the protein in hemp will be "left over" and unused because it is in an incorrect proportion.
This leads to another problem with hemp: it is relatively expensive. For example, I have purchased this hemp protein, Mantioba Harvest Hemp Protein Powder, with success and it is about $15-16 on Amazon (and about the same in my local vitamin store). This container has 15 servings, but the servings contain only 11 grams of protein. In other words, you are going to pay about a $1 per 11 gram serving of protein versus about 25 grams for some of the other classic proteins such as whey. When you consider it's low PCAAS score, it is a hard sell from the bodybuilding standpoint.
NOTE: I have also used Living Harvest Organic Hemp Protein and it is a little cheaper (~$11), but my point is still valid.
Does this mean one should not consume hemp for muscle building purposes? I wouldn't go that far: hemp does have some L-Leucine (.84 grams), the key , and so can only help. However, I would look at hemp more as a fantastic food that has some supplemental protein to help in your efforts.
One last thing I should mention: hemp has a rather strong taste. In fact, when you first put it up to take a drink, it smells like grass clippings. After awhile, you get used to it, but it's a far cry from some of the tastier whey products out there. Also, hemp protein comes from hemp seeds and these have the consistnecy of groundup bark.
Is it worth it? Again, it has many excellent properties that will help with long term health and bodybuilding, but it simply cannot be a core component of your protein regimen.