AFAIK VMWare utilise it's own filesystem vmfs for performance and efficiency reasons.
What will that be if I host my VMs on an Openfiler server which setup as NAS using ext3 filesystem?
Do yourself a favour - if you've got an OpenFiler box, make use of the iSCSI protocol. It's much more efficiant (especially with Jumbo Frames) and easier to maintain, and then VMWare can format the iSCSI disks with VMFS, and you get all the benefits of shared storage.
iSCSI isn't absolutely faster, and with FreeNAS I'd bet that NFS and iSCSI are equal in performance. You can also use jumbo frames for NFS.
As for ease of use, VMFS adds another layer of complexity. NFS is already a file sharing protocol and can be used for all the features that VMFS would be used for, and eliminates maybe of the limitations of iSCSI (volume size, number of guests supported etc.)