I've installed Google's Mobile Maps on my Motorola L6. It's a Java Applet (with versions for Windows Mobile and Palms also available), though it looks like there are tailored versions of the applet for different phones. Officially, it doesn't support the UK ("Now you can use Google Maps in Australia, Canada, France, Italy, Germany, the Netherlands, Spain, and the United States."), but it seems to work fine.
I'm not convinced by Google's directions - the route it gives between my flat and my parent's house is patently insane (the directions it gives will get you there, eventually). This isn't the first time I've had doubts about Google's directions (there's a section of the M25 it seems to completely ignore), so I doubt this is a problem only with the mobile version of the maps (and checking the route on the desktop version - it's the same route). I can sort of see how it goes wrong - it looks like it's calculating the shortest route (but even then, it's not the shortest possible route), the only thing I can think of is that it's the ideal route if there was no traffic involved. One day, I might try the route at 3am, just to find out.
There is one issue with the directions that is directly related to the mobile version of the maps. I stored the locations of both my flat and my parents house as favourites (I searched by postcode, stored that as a favourite and then renamed them to Flat and Home, respectively). When I try to get directions using the stored favourites, my start position is somewhere in Essex.
Other than directions of dubious quality, the rest of Google Mobile Maps works well. It has the most of the features as the desktop version, such as the satellite view - but there doesn't seem to be overlay mode. It can eat up data though (especially satellite imagery) - so it puts Orange's "all the data you can eat (up to 25 MB) for a £1" to good use. It's easy to use, and both maps and sat images display well, even on the L6's tiny screen. Finally, and possibly most importantly, it's free (as in beer).