I do not know anything about Driver Update specifically. My guess would be that it is legit. But I really do not know.
I do know that there are driver update sites/programs that are legit. They look at the drivers on your computer, look online for the latest drivers available, and compare your driver with the latest to see if a newer one is available. You can actually do the same thing manually if you know how.
The one problem that I have come across with these is that the newest driver is not always the best. Some people have installed a newer driver only to find out that it does not work for them. You just have to be careful and ready to undo it if something does not work.