The main issue I have with disk to disk backups is disaster recovery. Assuming the data and the backup data is on the same site, if something bad happens, like the building burns down, you've lost both the data and its backup.
At my place we've setup a load of disks in caddies, so we literally swap them round each day just like we would otherwise do with tape, thus getting the best of both worlds. In our case we chose disks due to the capacity versus cost implications, as at the time LTO1 was still new and expensive, and it worked out cheaper.
If you want the speed benefits of disk to disk, I'd recommend either doing something similar to what we've done, or use the staging backup feature, to backup the data first to disk, and then back that data up to disk, where the backup window is no longer an issue. You can then restore bits and pieces as required from the disk, and have a copy of the data on tape which can be taken off-site each night, or kept in a fireproof safe (though I'd personally prefer it off-site, rather than relying on being able to get access to the safe, while the firemen do their thing!).
