I think I speak for everyone here when I say that it is a somwhat difficult program to use. It requires A LOT of pre-planning and making sure that you completely understand Legato before you strart implementing it. Invest in a Legato consultant before doing anything with it. (Doesn't have to be Legato themselves, lots of other companies are able to provide support/consulting work)
The more complex your envirnoment the more issues you are going to have, but that is true no matter what product you use.
Legato support is shockingly bad, at best. The seem to be more interested in getting new customers then support exsisting ones (in my opinion anyway, so don't sue me)
There very few qualifies Legato support people so training/hiring of new staff can be a problem.
10000+ qualified BackupExec people vs. 3 qualified Legato people. Again, in my experience anyway.
Over all, it is a good product but only in certian situations.
Much in the same way Unix is a good OS, in certain situations and Windows is good product in certain situations.
Look into the product but be prepared for many sleepless nights.

good luck