I think mega garchomp is worth getting. When he megas, all of his stats increase except hp and speed. The hp stays the same but his speed lowers. I think speed doesn't really matter in this case. With a fast garchomp, you have much lower attack. I think if his speed stayed the same, garchomp would practically be overpowered. Garchomp's EV increased by 100 after mega - that's insanely good. If you really want a fast garchomp, try to get the nature "Jolly" for him. It increases speed and lowers sp. attack. It's great because you get a faster garchomp, plus he doesn't learn much special moves - more of a physical guy. Or, if this is too hard, give him a quick claw, so you can destroy your opponent early (if activated).
The point is, mega garchomp is worth having, if your a person who loves to focus on attack. In the end, you're barely losing anything.
Hope I helped! :)