I have had experience with other VTL's that use compression.You win space, but to keep your drives in streaming mode it sometimes becomes hard.For example , you have an LTO4 drive which requires 40 Mb/sec datafeed to keep in streaming mode.With Oracle or other databases, you can get really good compression ( 5:1 ) and higher,which will mean that you need a throughput of 200 Mb/sec single stream to keep your LTO going.If you do not get that,the drive will stop/start/stop/start ... which kills your tapedrive in the end.
rgds,
R.