We have about 3,000 K tapes in service currently within two atl's (one with
16 drives the other with 8)
One environment takes in between 700 & 1,300 GB a day (ok, average about 1
TB) of compressed client data
the other isn't as heavily used (yet) and takes in about 200 GB daily

In the past 6 months I think we had one tape's leader block flip off the
~little rod~ right as the tape was finishing a rewind so it didn't unload
the tape properly.

We've had the typical handful of I/O errors that resulted in a tape being
marked "readonly" but the tape was really OK...

Having good luck here...

I believe our tapes are the Imation black watch extended 3590's
        (we use Media Recovery out of Kansas City, Missouri... Tom McHugh
tjm AT mediarecovery DOT com)
        (sorry for what sounds like a sales pitch but this guy works hard
for his customers, when in a crunch, he comes through)

Dwight