Jirka - This app really moves things in the right directions. It really feels robust and well thought out. The attention to detail is great- the documentation is good and helpful, things like the little scripts to launch the app at the right size show that the developers care about ease of use. Having seen Ondra at work a little on the PSUMoni app I know that the quality of the programming will be spot on too.
On the constructive criticism side - I see that you mention the original colour scheme, to be honest the scheme does look a little drab but I don't think it is much that a little "aquafication" and some transparency etc. wouldn't address. Might it not be a good idea to hire a graphic designer to spritz things up a little. Or you could do what others have done and make a call for design input. I am talking purely about the cosmetics here - I know that the "bones" of the system is amply catered for by the programming team.
I, probably in common with others, will use the Mac primarily for two things 1) GPS 2) Media playback, mainly audio. As I don't yet a GPS puck (but hope to do so soon) I can't really comment on the GPS function yet. However I do have a couple of suggestions on the media player front.
1) It takes a loooong time to navigate to a band or song beginning with S or T from the start of the list. Many people will have lots of songs on their hard disks. Might it be a good idea to have the ability to jump directly to the points in the list beginning A,B,C,D,X,Y,Z etc.?
2) It would be really nice to have some album art. It just makes everything more intuitive and looks a lot nicer.
3) For a lot of the QCar modules the main window size is fine but for the music player, and perhaps some other modules, the main window is just too small. I'd question the philosophy of keeping all the other modules visible and thereby limiting the useful size of the main window. I think I can follow the rigour of the approach but it results in a very small usable area for some of the modules. I think it might be better to allow every app to be displayed full screen if necessary. Since the majority of people use a 7 or 8" screen I think you need to maximise the use of the screen real estate.
I know its early days for the cosmetics but I think many of us like the in-screen display to look cool and most people that use a Mac are distinctly design aware. I think it becomes all the more important that the system looks, as well as functions, good as you are planning on selling the application.
All in all though I think this will be a winner. And I can't wait for the Phone integration module.