"I would consider using ext4 on brand new installations since the performance increase is pretty significant. I believe that ext4 is default for Ubuntu on the next release (9.10), but available but not default in the current release (9.04).
You can convert a filesystem currently as ext3 to ext4, but the performance increase may not apply to the existing data since some of the internal architecture changes would not apply to that data. Any new data though, you would benefit from extents and such.
ext4 is marked stable, so releases of new kernels should not completely break your existing filesystem like an unstable filesystem like btrfs would.
Of course, I'd back things up before messing with anything, just in case....you never know."
