Hi Wolfgang,
I hope I'm not too late, but development here is faster than I can read this list. :-)
No, you're not too late - it's still "early days" for the inter-tile routing.
I made a test map of Germany composed of rectangular tiles from europe.osm using r918. Routing works fine on my StreetPilot c510 for almost any destination and the chosen route is always reasonable. (Routing in MapSource is unusable, but you knew that already.)
The only case where it doesn't work is when the destination of a long route is in a tile corner at the map border. If you start closer to the same destination it is able to find the route. I would guess that this is simply due to the cuts in the road network near the destination and Garmins preference for highways over long distances.
Well, that's the kind of feedback I like. Sounds like it's working well for you. If you want to test another new feature, there's a patch (posted yesterday) that provides support for turn restrictions. It's not yet checked into the SVN but if you can apply the patch to the latest SVN code and try it out, I would be grateful. Cheers, Mark