Much higher performance for large directories, even compared to other balanced tree filesystems. Partitions can be resized while in use. Rather than taking many seconds or even minutes to check the filesystem as e2fsck does, ReiserFSck takes only seconds. You may want to hold off implementing in production systems, although results to date have been good. Kernel-based NFS is not very stable yet.
Patches are available in many cases.
To make the change, you need to back up your data, create the file system, and restore. When large writes are scheduled all at once, reads can starve. A fix for this is in the works, and the later your ReiserFS patch, the better this situation is handled. This utility is equivalent to mke2fs. It is equivalent to e2fsck on ext2.