AndNav2 is Android's first operational turn-by-turn navigation app, marking one of the first instances where Android's wide-open apps policy has actually put it at an advantage over the iPhone. At least, in Europe.
Since the software is based on the OpenStreetMap mapping data, the app will be more useful in some areas than others—map information is, at least in part, crowdsourced like Wikipedia. The app itself, though, is polished. The search and directions functions will be familiar to anyone who has used a satnav unit (or even Google Maps) before, and the turn-by-turn functionality seems solid.
The main issue with AndNav2 is availability, as their site lists versions for Germany, France, Denmark, the United Kingdom, Spain, Switzerland, Italy, Austria and Ireland. This could be an exclusion based on insufficient mapping data for the country, but in any case there's no reason that you couldn't install the British version and at least try it. The alpha release of AndNav2 is available here. [AndNav via MobilitySite]