Some people have great experiences with NFSv4. My experience has been that it's easy to encounter obscure bugs.
About a month ago I went through a period in which running emerge on any client on a network in which /home and /usr/portage is hosted on an NFS server would randomly trigger lockd errors on all the other clients, requiring a hard reboot to resolve.
Then after a few weeks the problem went away. I'm not sure which update (kernel, nfs-utils, portage, or some other dependency) resolved the problem. I didn't change any configuration during this time. That basically describes my experience with NFS - it's good when it works but it's also prone to mysterious and inexplicable problems from time to time.