If I understand the system correctly (but I could be wrong), the whole point of SBD is that it should always be worth around $1 and/or at least should not generally go much below $1. If it's valued around $1, then it means it's actually working as designed!
In that regard, it's different from most other cryptocurrency where people buy them because they think the value may appreciate over time. If that's what you're looking for, and want to invest/speculate on the growth of steemit platform, then STEEM POWER (or just STEEM, but this is not quite so good) would be what to consider. But if you hold steem/SP then your investment will have the possibility of going down a lot as well the possibility of going up in value.
In fact that was the whole point of SBD...you should be able to keep your money in it and avoid much of the risk. But beware that the fair value of SBD is supposed to be exactly $1, so any time it becomes worth more than this it is quite likely that over time it will correct and trend back to being worth around $1 again! :)