I've noticed by reading back over old posts that I tend to be less than enthralled with just about every cover crop the first time I grow it, so take this with a grain of salt, but...I'm less than enthralled with sunn hemp.  Here are the negatives that jumped out at me this summer:

• Japanese beetles adore the plants, which meant I either had to extend my beetle-plucking to the cover crop beds (and cover crops are supposed to be no work) or let our beetle population expand.
• Sunn hemp doesn't cover the ground quickly (or, really, at all) since it grows up instead of out, meaning that weeds pop up in the bare soil around sunn hemp's feet.  I didn't pull them because cover crops aren't supposed to have to be weeded, so our forest garden has a lot of tall weeds getting ready to go to seed.
  
• I don't feel like I got nearly as much biomass production per unit area with sunn hemp as with sweet potatoes, and sunn hemp didn't provide an edible return.
  
• Sunn hemp can't handle the more-waterlogged parts of our garden (but, then, only oats and rye have thrived there).
  

All of that said, sunn hemp is pretty, especially now that it's starting to bloom.  And the plants are legumes, so they fix nitrogen, which my other cover crops don't do.  Still, they failed to meet my two primary goals for cover crops (maximum biomass production and weed suppression), so I don't think we'll try sunn hemp again.