Gnome shell is not considered stable yet and it WILL have problems. I have not personally had a chance to run it yet, so I only know what I've read from others. With that said, are you disabling compiz before you try running Gnome shell? You can do this by going to System->preferences->appearance and disabling the desktop effects. Once you're sure that you have compiz disabled and have saved any work that you don't want to risk using you can run gnome shell by typing:
Also, if you ran into any errors while building gnome shell that could be the problem too. Did you install it by adding a repository and using apt-get or did you do it from source?