If you're talking about serious investment, move this to Securities section.
If you're willing to take a risk with investment on any of these sites, ensure 1 thing: transparency. If they profit model is explained but details behind not shown, don't do it. The major risk with any of these, even the legitimate ones, is that you're leaving all of your funds in complete custody of the service. If anything goes wrong, if they decide to just exit scam or block your funds, there is nothing you can do about it. Understand this well.
Personally tried Poloniex lending, I agree it's quite a good way to build a small income if you've got Bitcoin lying around you're not using. I've made a little but since withdrawn everything since I really don't trust exchanges and Poloniex has a reputation (as do all the exchanges sooner or later) of simply ignoring customer support if anything goes wrong.
I invest in a few casino bankrolls. Transparent with funds and I like the concept and house edge logic over long term means you're likelier to profit than lose (very long term).
Pure investment you can control? Bitcoin I guess
