I used Maploader months ago (just after free Ovi maps for S60 was announced) to squeeze the whole of the UK onto my N900 and it worked first time, so it's not true that it doesn't work at all. I used a guide from the forums here - no renaming or extra faffing necessary.

Using:
Windows XP
N900 with Global Firmware

Do those who it doesn't work for have Nokia PC Suite installed? I'd imagine the phone wouldn't be detected in PC Suite mode otherwise as it requires some drivers.