Yes you can leave it in and it will charge it up until fully charged, and then the unit will not charge any further. In fact, Garmin recommends that you have it plugged in all the time while you're driving, to keep the unit fully charged.
Newer Garmin units have larger screens, faster processors, deal with bigger map databases. They are set up to be plugged in. 25xx series dispensed with pedestrian mode, presumably because of the shorter battery run times.
I can't seem to charge mine in the house. Does anyone know how? It just says that it can't charge using the usb cable when it's either on my pc or using a usb plug