Very much beta
Promises a lot but alas doesn't deliver. First installation attempt on my Orange c500 locked the phone solid, on second attempt realised I needed an active internet connection for the program to run (won't connect to GPRS automatically). Initial map download was 1MB (thankfully over ActiveSync) and placed me somewhere in Germany (I'm not). Woefully slow, user interface is exceedingly laggy and keeps wanting to talk to my (non-existant) GPS receiver. The UI is also rather non-standard (for Windows mobile anyway). You can't seem to enter text using the keypad but rather have to use an onscreen keyboard navigated by the joypad - sucks! And the operation of the soft buttons seems to be very inconsistent.
Managed to supposedly get the UK maps on my mini-SD card but doesn't seem to include gazeteer so any location lookup requires data connection too. Large chunks of maps don't seem to be displayed and scrolling causes weird screen corruption. And it also seems to keep the backlight on even when I quit the app (trip to settings...power management fixes that).
Gave it a spin on the Smartphone Emulator and while the speed and responsiveness improved the stability didn't. My advice would be to try it out on the emulator first (free dowload from Microsoft) before letting it anywhere near your phone.
If I was feeling cynical I might suggest there's some dodgy Symbian emulation layer knocking about...
In summary: I'm sticking to Google Maps