I've been trying out ext4 since a couple of months right now, specially since it was renamed from ext4dev to ext4, but nowadays I don't advise anyone to use it, at least until kernel 2.6.30 or so  under severe risk of massive data loss!!
The problem I had comes from when I changed several kernel option wich made my system completely freeze on shutdown without properly syncing disks, and I couldn't even reboot. fsck didn't help me either.
Read article  for more information, and take it into account before trying out. As a side note, it is "safer" to have ext4 on a NOT SYSTEM partition (safer does not mean safe), but not on such an active partition such system one.