It's not executable software, just map data. Those .img files are a standard format shared by the current and previous generation of mapping Garmin GPSes. Various people have created Garmin map images from the free (as in freedom, not beer) Openstreetmap data, with their own tweaks of what's shown and how it's rendered to suit different purposes.
Other than size limitaitons, compatibility isn't usually a problem - the maps should work equally well in all units (though some may be faster to render/route than others). The main drawback of OSM derived maps is that coverage can be inconsistent - typically *extremely good* in urban areas (down to marking the silly gates on Sustrans paths, and showing where the steps are on my local university campus), but for example country lanes that haven't been explored by an OSM geek may be absent. There can be subtle flaws in the routing data that mean a Garmin set to automatically find a route from A to B will go via point D because someone forgot to 'connect' the two roads at junction C. This is academic if you intend to follow a track 'manually' with the map as a backdrop (a preferred method on many audaxers).
Garimin's City Navigator maps are more consistent, and tend to route more reliably, but only really cover proper roads and may be more out of date than Openstreetmap. Their Ordnance Survey derived topo maps can be nice for walking, but less useful for road navigation - too much of the wrong kind of clutter makes for a crowded display.
I think it's fair to say that Openstreetmap is now "good enough" for most purposes, unless you're somewhere with really patchy coverage, or intend to mainly use the unit in idiot satnav mode.
