Garmin GPS devices
The maps are compatible with nearly all Garmin PNAs with a color screen (e.g. Legend/Vista HCx and Cx, etrex x0, 6062/76CSx, Oregon, Colorado,Edge 705/800, Dakota, most Nuvi, some Streetmap).
Garmin Mapsource 6.16.3 or higher (Windows based PC/Wine/ Mac) Mapsource compatible – If you have not yet installed Mapsource – here is how to do it: http://velomap.org/tutorials/install-mapsource/
Attention: Mapsource versions before 6.16.1 do not show the map correctly and miss a lot of ways and streets.
Attention2: Mapsource does not show contourlines correctly.
Garmin Basecamp – Perfectly compatible starting with v3.
Qlandkarte GT (since version 1.0) – Cross Platform www.qlandkarte.org – no autorouting support though. Comes with most Linux distributions, binaries for Windows available, other platforms need to compile. I recommend Qlandkarte 1.0 or higher (go latest version!)
PDA, Smartphone (Windows CE or ME) Also compatible with Garmin Mobile XT (installable to any Windows Smartphone) – Garmin Mobile XT does. If you don't want to buy Garmin Mobile XT there is also Glopus (as of version 2.02 beta or higher) and Qlandkarte M which you can try. Neither of the latter supports autorouting however. Some POI are not shown on Mobile XT. Areas and ways/roads/highways are all correctly shown.
Incompatibilities: Iphone, IQue 3600 (and others?), old Garmin handhelds without color screen and (micro)SD card. Any other program or PNA that doesn't support .TYP-files. (like TTQV, Compegps,….)
Oregon / Dakota / 62 series / edge 800 and streetname bug:
I spent a lot of time to find out what exactly could cause the Oregon (if no beta firmware installed) or the edge 800 to drop streetnames (or more correctly the labels of it on the devices). After having another idea, I got it solved by providing a lot of testmaps to User Ritzenflitzer on mtb-news.de forum.
The problem is related to the fact, that the devices stop showing any streetname labels, if one of the types 0×00 to 0×07 (and 0x0b) labels are set to invisible. This however not only to the map that is currently active, but to all maps on the GPS, and only showing up after a hard reset.
Therefore here is what you have to do if you are plagued with this problem. Only use openmtbmaps from May 6. 2011 update onwards on your GPS and do a hardreset to get the maps shown with streetlabels. However watch out, there are plenty of other maps that can cause this problem. Most notably the Garmin Topo Deutschland 2010 as coming preinstalled on microSD memory card (the dvd version is fine though with default typfile). There are several other (mainly OSM based) maps that hide labels of theese types, so be careful, if you include any of them on your GPS, the streetnames will not be shown (sorry, cannot tell you which, just try it out yourself ).
Note that this bug happens with Garmin maps too (like Topo Deutschland 2010) and is a firmware bug, that hopefully gets fixed in the future.
Address Search:
No seraching for housenumbers yet (though you have to enter a housenumber). I recommend buying and installing Garmin City Navigator maps, to have working address search on the GPS (you have to deactive the City Navigator map though, so it is not used for autorouting or shown)
Not that some OSM based maps (those were you can search for addresses as POI) may run havoc and crash the POI search (definitely on etrex series). This happens as soon as the map is on the GPS, it does not have to be active.

I have just purchased a Montana 600 with Int’l basemap and UK 1:50 Explorer so if you have any compatibility questions feel free to email me. So far velomaps seem to work fine although I get a warning on Basecamp about inconsistent transparencies?
On Montana like all modern outdoor units by Garmin, they should work fine. Never seen a warning about transparencies though… Have fun….