shared file system

If you have multiple webnodes , you need some kind of shared filesystem to sync uploaded files, static files etc.

Instead, what if you use a pull based CDN system. So all your files can be served from cdn1.yourdomain.com and all webnodes point to CDN.
the benefits are
1) You don't need to worry about managing a shared file system
2) You also get a free frontend to server static files.

This looks like a much tempting option than shared file system. Do you agree? Or there are gotchas I am not thinking through?

We run a large Pressflow site with several webservers (load balanced with HAProxy). On every server the pressflow /files directory is shared via NFS3 to a master server. We see some limits in this NFS approach and would like to deploy a new, more performant and redundant system.
There are different choices like GlusterFS, MogileFS, NFS4, Lustre, ...
What do you use and how are your experiences so far with shared file systems?

Thanks for any best practice hints.