Dan,
VTL maybe a good solution in environments wherein restore and recovery times are critical. I also share Ken's point on what do you mean by "limited success".
If the original intention of having a VTL environment was to address SLAs around recovery or restore times, and you are achieving this goal, then I can conclude that the setup is a success. But if the intention was to eliminate tapes but speed up restores, and SLAs dictates otherwise - users and the business can tolerate longer data restore times - then it may be considered "limited success" or a failure at worst.
It all depends on what the business requires and the agreed SLAs.
For one, VTLs are not cheap to setup and maintain in the long run if the intention is to keep data over a considerable period of time. In my honest opinion, if data stays on a VTL environment for more than fifteen business days, the solution is not anymore cost effective. But this must be proven quantitatively. All indications shows that the more servers defined on a VTL solution, the higher will be the operating and maintenance cost.
VTL solutions must consider what the business needs, risks and goals are. If the business cannot tolerate long restore times, then VTLs maybe the way to go. Moderate restore times maybe addressed through a combined VTL and tape solution.