OCFS2, in the v1.4 branch has some extreme fragmentation problems. It seems to be just fine if you're dealing with large files, or lots of small files that don't change frequently. However, if you've got lots of turnover of lots of small files, you're going to get bitten.

If that describes you, you need to be willing to track the latest kernel releases, and able to rebuild your FS semi-frequently.

(I'm being bitten by this right now, and I need to rebuild the FS... not something I'm happy about.)