We converted our environment from VTL to NFS with Data Domain about 3-4 years ago, and it has worked very well for us. All our TSM servers are Linux, and we have a 10Gbps network infrastructure. Prior to this change, it seemed the majority of problems we had in the TSM environment were related to virtual tape, much like physical tape problems. The only gotcha I would look out for is that you don't exceed the maximum ingest streams for that model of DDR. I think it is 180 on the 4500, but not positive.
The only place we still use VTL is for the iSeries (AS\400) because it doesn't support NFS.
-Rowl