It is in Ubers, if that means anything (I know your not competitive). Ubers is the top tier (Basically), so I'd say it is one of the best mega evolutions. (Those including Mewtwo X & Y, Lucario, Blaziken, Kangaskhan, Mawile, and Rayquaza).
Discussion about this is more that teams have to be semi-specialized to fit a mega Pokemon in it, especially since there aren't as many options, and that Z-Moves can be on any team (even though you can have both).
And Hey, if a pokemon you like isn't competitively viable, who cares. That shouldn't matter to you.
P.S. Mega Gengar was my Mvp for my Pokemon X playthrough :P