Sorry Karam, I've missed your question about ext4. As I don't want to hijack this productive thread, I'll just drop link for consideration:

http://wiki.maemo.org/Changing_the_eMMC_filesystem

Near end of this article, some benchmarks are shown. I would also mention more reliable journal implementation, and all fixes/upgrades, that one can found by watching ext3/ext4 changelog Long story short, it works great on N900 with kernel-power, stock kernel doesn't have modules for ext4 AFAIK.