How does the mobile experience compare between Germany and Nigeria? There is currently no public data or test-bed to provide an answer to this question. This is because deploying and maintaining such test-bed can be both challenging and expensive. To fill this gap, this paper proposes a novel test-bed design called "AmiGo", which relies on travelers carrying mobile phones to act as vantage points and collect data on mobile network performance. The "AmiGo" design has three key advantages: it is easy to deploy, has realistic user mobility, and runs on real Android devices. We further developed a suite of measurement tools for "AmiGo" to perform network measurements, e.g., pings, speedtests, and webpage loads. We leverage these tools to measure the performance of 24 mobile networks across five continents over a month via an "AmiGo" deployment involving 31 students. We find that 50% of networks face a 40-70% chance of providing low data rates, only 20% achieve low latencies, and networks in Asia, Central/South America, and Africa have significantly higher CDN download times than in Europe. Most news websites load slowly, while YouTube performs well. We made both test-bed and measurement tools open source.