Yes - useful for backup, and, it has to be said, hard to go back to the dim Garmin screen after playing with one of these shiny things for a while - but battery life is a major problem.
Currently looking at the HTC Wildfire.
We went shopping for a Wildfire for Sheila but eventually walked out with a Desire, and I don't regret it. It's all about the screen with these things, so a bit pointless not to go the whole hog IMO - and the keyboard is a bit of an issue on the smaller Wildfire screen.
Even the latest Google maps seems only to cache a rather limited amount of map on the SD card, so I put
Alpinequest on, this uses maps stored on card (it can download 'live' maps as well, but it isn't the default) and is nice to use in a quirky, androidy sort of way. Easy to load and display tracks, easy to record current track, probably not up to more advanced navigation tasks.
Sheila's touring in India at the moment so Gmaps isn't really an option - the offline maps are essential - the Garmin is for on-the-bike use, but Alpinequest is very nice for browsing ahead in the evenings.