For the most part so far, we've used the iOS simulator to test the mobile version of our site. That's pretty good, a lot better than testing a narrow browser window, but it is also not nearly comprehensive.
We use BrowserStack to test the site on some other devices, through their official simulators. Everything was looking good on the Samsung Galaxy III, but there were plenty of issues on older versions of iOS.
We also look at Parallels, which I've started using again to test Windows stuff just because it's so much faster than BrowserStack.