I abandoned Dell commvault support and went straight to Commvault themselves and couldn't be happier. At the time Dell tech support couldn't figure out if Commvault was a tape drive or a server and it was hell getting help from them on anything. I should add that this was at least 3 years ago, so things hopefully has changed.
I've been running Commvault for about 6 years now, and it works great for us. I haven't used TSM so I couldn't help you compare the two, but disk to disk and then to tape is what I've been doing for ages now and it works great.