I work as an Engineer at a VC backed ($20m from two rounds) product company in the US. We used outsourced Engineers for most of our developers. We are located not located in Silicon Valley but some of our VC's are from the valley.

Our founder & CTO has experience with co-founding another successful startup ($25-$50 million in revenue) with an outsourced engineering team, which I also and many of our team members also worked at.

The engineers we have employed in Kiev are very good. However, working with outsourced engineers is still very challenging. One of the things that allows us to be successful is our experience with it and what we've learned from our mistakes at our previous company. I wouldn't recommend it for your average company.