I can't add too much to the chorus of peoplewholike Super Duper as a Mac OS X backup utility, mainly because I don't use it that way. (For backups I faithfully use psyncx.)
However, I am a very happy Super Duper customer. I use it because most of the time my primary computer is a desktop iMac. But when I have a presentation to give, or I'm otherwise going to be out of the office, I use Super Duper to copy my /Users directory to a PowerBook. This ensures I have all of my files (and the different user accounts that I use) with me when I need them. When I get home another Super Duper session back to the iMac and I'm once again current and have all the changes I made while on the road. Because I have to do this somewhat regularly, I use the "changed files only" option to reduce the time it takes; but overall it's definitely fast enough and very, very convenient.
I know that I'm missing out by using Super Duper as a one trick pony, but it's a good trick that is essential for my current working style, so I'm happy. If you're faced with a similar need, I suggest you give it a try.
IMPORTANT UPDATE. DEC 2007. - SuperDuper is not compatible with Mac OS X v10.5 Leopard. Despite knowing about this before Leopard's release, the product has not yet been updated. Do not use SuperDuper with Leopard, it will be unable to correctly restore your files. It will, however, happily run without complaint while it screws up your system. I speak from experience.