I would be curious to know what the two other GPS you have.
I would agree that the basemap provided should not be used (under 20km).
However the Garmin maps I have are accurate detailed and simply excellent.
Cant comment on your other receivers having more accurate maps but If compared to City Navigator I suspect there just as accurate/inaccurate.
The service menu can be accessed by pressing enter with the power button on startup. It wont show the card ID though.
The only way I can think of to have a copy of a garmin SD card readable on a GPS is to open the .img file and add/edit the unlock code within.
I would look at getting a Garmin GPS 18 as you can find it with City Select or Navigator for around $100. If the version you receive is not v2008 you'll get two unlocks, with the "primary" unlock getting the free upgrade.