We generally hold a negative view of interest-bearing Bitcoin accounts, both for individuals and for the Bitcoin community. Such interest-bearing accounts are investments, and with any investment, you have the potential to lose all of your funds through mismanagement on the part of the custodian. Whereas if you hold your own private keys, you cannot lose it through the actions of someone else.
Any custodian who issues you an interest-bearing account has their own incentives structured to loan out more money than they take in. In short, they make more money when they practice fractional reserve banking. So unless they do a full proof-of-reserve on the Bitcoin blockchain (like Coinfloor.com does), you never really know whether they still have your Bitcoin, whether they've gone bankrupt, and whether you will ever get your Bitcoin back.
While interest-bearing custodians may have the morals and vision not to practice fractional-reserve banking the short-term, we believe that in the long-term people and companies follow their profit incentives and so all companies, no matter their reputations or histories, will eventually be disposed to put your Bitcoin at risk, over-lend it, or come to see it as their own. That is why we set Swan up explicitly to always store the Bitcoin in separate wallets legally in your name, so we never come to view it as our Bitcoin.
The more you learn about Bitcoin and how powerful it is to hold your own keys, the less likely you are to want to give up ownership of your Bitcoin to someone that promises a mere 4% per year, especially since the value of Bitcoin will vastly outperform 4% annual growth.
If you are still interested in earning interest despite all these risks and despite giving up the freedom and privacy of holding your own keys, we recommend a very small allocation (<20% of your total Bitcoin). We also recommend that you periodically remove your Bitcoin from these accounts to test the companies and remind them that it is your Bitcoin, not theirs.
If you are interested in other bitcoin investment sites, read here.