I had a few bucks in btc-arbs.com for a week, but the return really isn't worth the risk, and I withdrew everything this morning. They claim UP TO 10%, but it hasn't been remotely close to that ever. I think the biggest day they had was only 5%. At that rate (highest single day) of return you're looking at minimum of 20 days (realistically closer to 40+ days ) to gain the interest of your initial investment.
That's what makes them more legitimate, in my eyes...not every day is going to have a great opportunity for arbitrage. However, they DO use the days (like today) where the exchanges are very close to each other, to rebalance their accounts and prepare for the high volatility days, without needing to wait for wire transfers. So that ends up being a positive. I have seen them miss the chance to capitalize on opportunities because a bunch of cash was tied up in wire transfers.
Where else can you put your Bitcoin where you'll see any kind of passive return? Even if you only managed to average .5% a day, that's better than any savings account you'll find.
correct, i'd be happy with a steady 0.5% interest. The only thing i am worried about it their legitimacy, I risked a small amount of btc just to see what happens but i would not advice putting money in you can't afford to lose.
There's no guarantee that they won't run with the money.
But I've got money in the pot too and have had no hitches over the last month and a half with deposits, withdrawals, or earnings. Doesn't mean it's not a scam, but no one can definitively say it is either. 
hope they'll prove me wrong