We have 2 3494's in house, attached to OS/390 for use with DFHSM (soon to be
used with a TSM server as well).
The oldest is 3494 is 2 years old.  We also have STK 9710 robots in house
for use with TSM on AIX, so I've worked with both.

The 3494 isnt' quite as fast as the STK robots, but it holds more
cartridges.  We have had one minor alignment problem with the 3494 that
caused some occasional strange behavior, but the problem was fixed within 2
days when we reported it to the CE.

We don't have any serious load on the 3494's, they are used all day long,
but we've never stressed them   They were installed here primarily to reduce
the mount of operator coverage required.  DFHSM backups (which use the
tapes/drives very much like TSM does) now run unattended overnight.  We
don't have any of the high-availblity features except the second hard disk;
no dual gripper or accessor.  Have never needed them.

We have no complaints at all about the 3494 hardware, and I don't think you
would ever be sorry you got one, it's rock solid.  If you have any
difficulties, it would probably be with the interface to VM, I have no idea
how that works; the interface to OS/390 is a little difficult to get used
to, because the documentation is somewhat lacking.

The best documentation is the red book, SG24-4632.  I think it actually has
some information about performance & mount times you can expect if you have
dual gripper vs. single gripper, stuff like that.