I recently works on a merb project. As a vim user I found merb.vim for syntax highlighting, but it will conflict with rails.vim. For example, *before_filter* is a declaration in Rails controller, but in Merb it should *before*. After you installed merb.vim, it will set the file type of your controller file to ruby.merb_controller, so if you open a controller file in a rails project, the 'before_filter' will be highlighten as an error.
The solution is to find a way to have directory/project specific setting for vim. Luckily it already exists.
In you ~/.vimrc:
Then move ~/.vim/ftdetect/merb.vim (this file is installed by merb.vim plugin) to $your_merb_project/.vimrc. Now the filetype setting for merb will only affects the merb project dir only.