Open files (as in the operating system) seem to get backed up fine.
Open files (open by an application, e.g. AV software) have been a persistent problem with tape backup apps. AV software sometimes keeps them open so that they cannot be messed with...
NTbackup (as I think it used to be called) is just a cut down version of the ancient Seagate / Veritas / Symantec / whatever-they-are-today Backup Executive program. It even used to be tape format compatible with the real $1000 thing. It probably still is - I will check a tape against BE10 tonight.
DLT is fast! 10-20 megabytes per second, and the PC hardly slows down during the backup. This is SCSI at its best - IDE or SATA doesn't even begin to compare. The verify process is about 30MB/sec.