First, personally, I can’t be bothered about the license change itself. There is one thing that I want legally from OSM which is: it should reasonably protect itself against one-way copying. I.e. Google grabbing OSM data (which would be great), enhancing it using it’s own map community (awesome) but don’t provide updates back to OSM (which would be bad, imho). I am not in any way interested in the legal details and if the ones that do decide that CC-by-SA is not working and ODBL is better then I accept this. All I’m interested in is knowing that the new license will offer equal or better protection against one-way copying.
I talked a bit with Jordan (the lawyer behind ODBL, afaik) at SoTM Amsterdam and concluded that, basically, although we used English, we don’t speak the same language. I’m a techi but licenses are on lawyers turf and I don’t have anything to add to their discussion, unlike other aspects of OSM where I do have knowledge and an opinion… 
I’d like to plead one thing to the people who have not and intend not to accept the new license because they have problems with the new licence or the change or the way the change is handled:
We have (long) worked together to get OSM where it is today. You have probably spent many hours on mapping as did I, continuously improving each others work. If you decide to leave OSM maybe continuing mapping somewhere else that is you choice, I accept this ofcourse, it’s sad that it has to come this far, but I understand that not everyone is as indifferent to the license change as I am. But please do accept the license before your leave so that your work and that of those who built upon your work is not lost to OSM and those who ‘stay behind’. Please don’t punish every other contributor for any disagreement you may have with a juridical sideshow.
So, back to Garmin routing. Even if OSM is forked I expect OSM to be the benchmark for the forked projects for a while. So every forked project will support OSM XML or OSM PBF output (which I’m using recently) and, thus, I expect the Mkgmap project to continuously move forward as it has been so far. So routing will become better, address search will become better, sea polygons will be enabled one day, etc. The source of the data should not really matter.
I must add: I’ve started mapping with OSM because I wanted free Garmin maps. OSM has jumpstarted the whole wikimapping and it deserves a lot of credit for that. I won’t quit mapping with OSM and I will continue to promote OSM through Garmin maps. If the license change temporarily reduces the map quality then so be it. Maybe I won’t update the map for a while until the worst damage has been repaired or I’ll provide two versions (an old stable version and the latest), I don’t know yet.
If someone wants to run his own copy of my Garmin website, he/she is welcome. Send me an email and I’ll provide a copy of the scripts and (limited) support to get things running. Keep in mind though: you’ll need significant resources to handle a popular site like this. Don’t think of this lightly and don’t ask for the scripts if you’re not really serious about running your own copy. I have better ways of spending my limited spare time…