I'll third it too. I visited earlier this year with a UK trade mission for the Maker Faire and to meet suppliers. I visited several factories, both with the trade mission and independently, and it was one of the most fascinating weeks of my life. One place that I'd definitely suggest visiting, if only for the contrast with most other manufacturers, is Seeed Studios. Really cool place, and totally unlike any other place I visited. It's also very well set up for tours.
