I tried to work with other 3 people on a Unity project. Whenever someone worked in the project folder, the whole files were shared to the other ones exactly, via Dropbox. The issue is that whenever someone wanted to open an updated version of the project folder, Unity reimported all the assets, which is indeed very annoying and time consuming. Is there anyway to stop Unity importing the files every time someone modifies the project? BTW, do you recommend any DVCS or something like that solution that works well with Unity, other than the Asset Server? Thanks 
With Unity Pro, any VCS, D or otherwise should work fine.
With non-pro, you can only have one person modifying the project at a time (to the resolution of the sychronicity of their computer clocks). Depending on exactly how you synchronise your files, you may get other problems. For example, if someone is actually copying the whole repo, all the timestamps will get smashed, leading to a complete reimport.