I would agree that Filers work well for databases, but I would not limit that to DSS workloads. In fact, I would say that Filers offer tremendous benefits to OLTP environments. Particularly performance and admin features.
As for performance, everyone has an opinion, and every application will re-act differently. It is very difficult to make a blanket statement like "it only works for ...".
I am with NetApp, and you were probably looking for shall we say, less biased input. However, I felt compelled to respond. I hope you will indulge me.
I have used Filers with Oracle while working at Oracle, and had very good performance. I will let others attest to their own experiences, but I will point you to a document comparing perf. of locally attached FC disks with a Filer - http://www.netapp.com/tech_library/3105.html
I am not sure what you mean by NetApp comes right out and says that they don't recommend using their box in OLTP configurations. I don't know of any recommendation like that.
The biggest reason is that OLTP database tend to be lots of little
transactions and since you are using NFS (relatively big transactions) you won't get the performance gains as you would with a DSS config.
Relative to what? Databases usually deal in block sizes of 4K, 8K or larger. Since the database block is the smallest amount of data that would be read or written, I don't think that NFS would cause an undue burden. In fact the NFS frame size can be set to match the database block size if that were necessary. But in fact, most databases will do read ahead (like the multi-block read ahead in Oracle) so that larger read sizes could benefit the database even in an OLTP environment.
-tom