They are nice and fast, I think if you want to run one though, you really have to run 3 to make them work.
They can seriously threaten a turn 2 charge but one is easy enough to shoot down. With 3 you can have them in an armies face and 9 AV12 HP's with a 5+ save [4+ cover if theres any ruins around] is pretty hard for a lot of armies to bring down.
I think triple maulers would work well by adding another fast threat on top of that, Nurgle Spawn with Bike Lord, Khorne Bikes with Jugger Lord etc.
What troops to throw in with that kind of build is an issue though, I suppose any shooting oriented marine squads sitting backfield are at least going to avoid taking any firepower for at least a few turns which will allow you to focus your fire on hopefully taking out whatever can shoot down your Maulers.