I'm going to upgrade from Ubuntu 9.04 to Ubuntu 9.10 in a near future and wanted to know if I should migrate my existing ext3 partitions to ext4 during the process and, if yes, why?
Ext4 Howto has some information about the features of ext4. Looks over benchmarks find ext4 faster than ext3, though it's not required to turn on all of the new features.
Honestly, you could mount your ext3 partitions with ext4 and take a taste of ext4 without needing to use all the new features. You'll have the ability to go back to ext3.
Ok, according to this blog post introducing new features of Karmic Koala:
So, I won't have to choose during the upgrade from Jaunty to Karmic itself. That's actually a good thing.
Then, I followed the given link describing how to upgrade to ext4 manually and, having read the whole post including comments:
I will thus patiently wait before to upgrade to ext4.