I've not yet swapped out my LandRover SatNav unit for an android device but will probably do so soon.

However I have use android devices for a few years now to SatNav and thought I would share a couple of pieces of software that I feel are really good:

(1) Igo Primo 2 - for me probably the best SatNav software for android. Easily customisable and a really really good display (3d building, landscape etc). Knocks the spots of TomTom in my opinion. The actual navigation itself is also good. The main issue is that there are two types of maps available. One is based on TomTom and the other on NavTeq. The TomTom ones rarely get updated whilst the NavTeq don't seem to hold as much information. But both are excellent for main road navigation.

(2) Ulysse Speedometer - a really cool and free speedo app. Gives you that digital speedo look if you load it up when not using sat nav

Nice tips - thanks.

If you ever venture off the tarmac in the UK (either on green lanes or, assuming your device isn't build in to the car, on foot, cycle, horseback, etc) Viewranger and Memory Map are also worth a look. Both will give you GPS with OS mapping so you can make sure you don't stray off the right of way. They don't give voice directions (not that kind of GPS) but you can create waypoints and routes to follow on fully-detailed OS maps. Viewranger was my choice of the two, and I'm happy with it so far.

Another option for off road mapping using OS maps is OMN by Anquet. Data can be sync'd between your android device and PC, waypoints and routes created. As with Memory Map and View Ranger, there's no voice guidance, but to be honest if you are somewhere that you need this kind of mapping, then you probably don't need it!