I am currently testing mhvtl with NetBackup 6.5.5 and having a great success so far ..
I am using mhvtl 12-2-2009 release running on Centos 5.4 32-bit kernel 2.6.18.
Not a single issue as or right now ..
All acting as if it was the real thing..
I will continue to test and provide more feedback.