We have a clustered F880 with about 8.5TB of usable data on it, split among a dozen volumes.

FWIW, we back this up onto three LTO2 drives in about 60 hours (i.e. we start it at 7pm Friday and it runs until Monday morning). This occupies about 24 tapes.

We originally wanted the 9940B, but it was too expensive, and I have to say that I am very happy with the LTO2. I have not had a single media failure among the nearly 500 tapes that we've used. They are very fast and very reliable.