Datadomain makes one of the best deduplication appliances on the market today and for the last 6 years. They were the first, and still the fastest.
The DdataDomain (DD) appliance comes in varying sizes and speeds, with other options like replication, OST/BOOST, encryption, 10GbE, quad Gb NIC's, etc...
It's important to know this is a backup target. It supports CIFS, NFS, and VTL. So if your application can natively dump a backup file to CIFS or NFS, you're golden. If not, you'll have to use a Backup application to write to it.
BackupExec's as well as NetBackup's dedupllication options, offer a lower cost solution to dedupe, that is still rather new and has a bit of teething issues. for example, their Vmware dedupe performance is dismal in BackupExec and wont be fixed till R3 they say. But other things like flat files dedupe well. If anything, NetBackup has a more robust dedupe option IMO.
While it's lower in cost than a DD appliance, it's also not as good in terms of simplicity, complexity, and support. But if you are on a budget, and "good enough," is sufficient to you. Great. But if you need solid predictable performance, and reliability, than you may want a combined solution of a DD appliance with a soild backup product like BackupExec or NBU