we've had awful problems for 3 solid years with 4.5FP6, we went FP8 in April this year and excepting 2 write errors (tapes since destroyed) have (touch wood) not had one single blip with NetBackup in 3months. A record for me as i've been using NBU for 5yrs since version 3.1.1 and have never gone this long without error!.
We are looking to go 5.1 to get some disk staging functionality but as ever i'm very dubious about upgrading as it's a lottery. We've spoken to some Netapp guys and hear that 30% of Netapp code is in 6.0 BUT there is no way we're gonna go near 6.0 for ages after as we simply can't afford to be at risk of bugs/'features' with our backups.
ps Don't get me wrong i love some of their products - NBU especially, Volume Mgr, File System, Flashsnap especially.