A few days ago I read about this Crypto Lend service in passing on reddit. Personally, I find all the exchange funding pages to be very complex, and the chart of lending/borrowing is always changing, so I am happy to let a service do this step for me. Obviously I am very pleasantly surprised to find that the service is free, and by using API keys through exchanges there's no risk from the service itself. I've got to say I am really impressed! So far I've only put 2.5 BTC on exchanges to be lent, but it has been lending and working well for me. At first I was under the impression that there was an element of arbitrage, where the cryptolend service would find the best rates and move BTC between the exchanges. But that's not the case, you put your money on the exchanges manually, where the Crypto Lend bot can see it at which point it sets the bitcoin to lend though that exchange's API.

Bitmex:
I'm a little wary of doing this on Bitmex because there are so many warnings and the concept of lending on Bitmex is complicated. Plus you get charged some sort of fees for the conversion/opening, I just have no idea what I'm doing there so I've stayed away.

Bitfinex/Poloniex:
The rates since I've started two days ago for Bitfinex have been less than half of Poloniex, so I've put a bit more on Poloniex. This point has nothing to do with Crypto Lend as a service, but just to say it's working well on both exchanges lending BTC.

Final worries:
Crypto Lend is awesome, I wish there was a way for you to take a % of the profits because that seems like some good goal congruence, but I guess without actually giving Crypto Lend control of the funds, that'll never happen fluidly (invoice people maybe? First 1BTC of profit is free, then pay $30?). Since my coins are usually just sitting around anyhow, I wish there was less risk to my cold storage if I were to put them on these exchanges! If I could have even the smallest guarantee or insurance against exchange hack/theft, I would let Crypto Lend take care of lending all of my funds.