Mega Pokemon are not necessarily all that strong. Their strength depends a lot on the original Pokemon. For example, in my opinion, Mega Sceptile is worse than Sceptile in ASB. Lucario and Mega Lucario are both great, but I consider it to not be that good of a Mega, because it is barely a boost over Lucario. You may be better off saving up for some other Mega.
Of course, if you like something, use it, regardless of whether it is good. That is one of the points of ASB, to use favourites. Slash is the Route 1 Gym Leader, and he has plenty of Pokemon that are not considered good, but he makes them work very well.