I don't know what you're general smash level is so there are a couple of options,
In general down b is a really solid move, you get 25 frames of intangibility, and a boost to wherever you're going. You're correct in knowing it can be canceled, but only with aura. When grounded you can do any grounded non special move, and in the air all aerials.
You can also cancel any of lucario attacks (if they hit your opponent or their shield) into specials such as down b. this gives you good in-and-out mobility as well as potential tech chases.
Down b also opens up some approach options as well, one of the best in the neutral is a down b cancel into up tilt, it covers a wide range and pops your opponent up into easily punishable positions.
Overall down b is a great move, it can really overwhelm your opponents and bait out punishable moves if timed correctly, I could go waaaay more into detail about it but I'll leave it here for now, if you need more I, or some one else can surely help you.