I ordered an author proof, which was printed in the UK. After fixing some issues (on my side) I ordered a second proof, also printed in the UK. That looked good, so I published. Then I ordered an author copy from amazon.com, which was printed in San Bernadino, USA. I thought the quality was a little bit lower than the ones printed in the UK (also took ages, really, more than 3 weeks before they got here). I then ordered a copy from amazon.de, which was printed in the UK again, and I must say I think the quality is just that little bit better. It also arrived within a couple of days, not weeks.
Quality of the US printed book is not such that I would really complain, but it's just not as good. The cover is slightly shifted, which leaves a narrow black band on the edge. I need to fix that by leaving more trim space, but my cover image was well within the template KDP gives.
Anyway, it was a learning experience, but overall I'm positive about the process and the results.