I am an outright advocate of DPM 2010, i love the product and have spent a lot of time pushing it and showing people the ease of use of the product and how powerful it can be.

One of things that is important when planning for a backup solution is the backup window

how much time you actually have to backup the data without impacting the performance of the system. This window is narrowing every single day with the more common mobile workforce and enhanced methods of application delivery and remote access…

DPM is a disk based backup solution which can then send it to tape for long term storage BUT the initial backup is done to disk. This is important as you are not waiting on the tape drive to write the data to the tape which although much better now is still pretty slow compared to a hard disk write.

Obviously how quick the data can get to the dpm server is limited to how fast the disks can write but more importantly it is limited to the network backbone

So i think this technet article is very useful for DPM bandwidth requirement planning and understanding your backup windows