Let me see if I can add my experiences and see if they are useful to you.
OSSV does not do well in a "bare metal" restore situation. You cannot easily use it to put a server exactly back to the point it was when it was backed up. However, I would recommend that you test the feasability of this before coming to your own conclusion. I have seen some articles out there (somewhere) that explain how to do a system state restore onto un-like hardware. OSSV does indeed have the equivalent of an "agent" but it is very light-weight. To do single file restores you can either access the backup volume via CIFS as you said, or via DFM/Protection Manager if you have it. If you don't have Protection Manager then managing your OSSV backups can be done but there is a lot more effort required to set it up and monitor it.
Restoring from SV is pretty much the same - if the relationship is managed via Protection Manager then restores are easy. Even if the relationship is not managed via Protection Manager you can use the Operations Manager interface to perform a single file restore. You can get as granular as you like.
Most of Netapp's data protection software has no inclusions for tape backup. In the case where your data is an offline media such as this you have to have some mechanism for manually restoring the backup to some temporary location and then making the data available via CIFS or NFS. In this case I don't know of any easy way to use PM to assist you in the restore - it comes down to whatever software you are using the write to tape. It is particularly difficult when working with the SM products since usually LUNs are involved and you have no choice but to restore the entire LUN from tape.
Here, I'm hoping that we can move entirely to a disk-based backup because having a hybrid environment doesn't work that well. We still rely upon a tape copy for DR purposes but in that case we are going to have many more challenges involved.
As far as management is concerned, each software piece seems to have some kind of quirk involved to have to work around. SME always seems to be less picky than SMSQL and SMSP. I have not used SMVI since version 1.0 so I don't know enough about that to be able to comment.
Hope this helps. Feel free to contact me directly if I can offer any other insights.
Cheers,
Richard