Have any foreign companies successfully applied? How do they handle visas, company location etc.?
500 itself has no problem giving money to international companies - however, most companies flip-up or establish in the US to make it easier for other investors. It does depend on the company though. Some companies come to the US to enhance their network, not necessarily to crack into the US market, so setting up shop here legally doesn't make sense.
For visas - they provide references, etc that are needed and are very quick and responsive about it. That said, they will not "sponsor" you to come in on an H1-B or anything like that.
http://500.co/2012/04/06/visualizing-the-last-18-months-of-5...
It's a great way for international companies to quickly get integrated into Silicon Valley.