On a slightly related note, this thread inspired a fresh look at what's available, which led to
GB Topo Maps for Android. I've no idea what the routing capabilities are like, they're not something I use much, but for walking/cycling mapping and tracking it's probably the best I've yet found, despite the odd quirk.
It has the Ordnance Survey Topo Map with outdoor detail and proper Rights of Way marking plus another dozen or so available maps, including Open Cycle Map, Open Street Map, OSM Landscape and Outdoors and offerings from ESRI, Google and Bing including satellite, and night maps, with simple switching. For walkers, footpaths are shown on crowdsourced maps which aren't on the OS, which can lead to well trodden paths, even if occasionally they go through barbed wire fences.
For a mapping geek like me, who likes to understand the landscape around (in or out of the city), being able to switch maps enriches usefulness. For instance this puzzling feature is only actually explained on one map.
There is also the (paid) option to add other map layers, so I added an
OS map in the 1940s which gives a wholly different context.
It also has built in overlays including hillshading, with the option to add more. I haven't explored data layers yet, so far I've just used pinned historic overlays. Sadly it lacks a transparency slider, even so I'm liking being able to compare.
The only real issue I've found is that caching for purely offline use is a bit odd, but generally for £4 a year I think it's a great app.