If everything goes absolutely swimmingly, you will not make more than 6.5 percent on your investment. They're not going to give the bondholders a tip/bonus/whatever.
If you believe that Tesla + SolarCity is going to be a powerhouse, I'd consider buying the stock instead. Alternately, if you're looking for a 6.5 percent return, there are less risky ways to get it (it's about the expected annual return from a broad index fund).
I'm honestly not sure who these bonds make much sense for. The implied bet is something like "they're not going to do great, so the stock will not rise, but they also will not go bankrupt so I will get my principal back." On the other hand, if you are emotionally invested in the idea of self-driving solar powered electric cars and are willing to tie up some money to make that dream come true...I can see the appeal. (I own a tiny bit of TSLA for the same reason).
Then why offer interest rates many multiples higher than other low-risk investments? Of course they might pay off (in fact, it's probable that they will). But it certainly isn't riskfree, like a savings account.
These bonds are medium risk... you get all of the downsides of when they go bankrupt but none of the upsides of if they become wildly successful.
Please forgive my ignorance on this. I think there is a high potential that SolarCity will do well after merging with Tesla so I see these bonds returning good returns.