I know people are now defending this thing because they want it to be good, but let's check out the facts.
M-Sceptile is weak to Bug, Flying, Ice, Poison, Dragon, and Fairy. That's six weaknesses, which is never good. This includes a weakness to U-turn as well as Bravest Bird. Sceptile is frail and it's also vulnerable to all hazards, so that only makes things worse.
>B-but it destroys Electric-types
Hardly. You can't just switch into a predicted Volt Switch because you have to Mega Evolve BEFORE receiving the hit. So basically, you Mega Evolve your Sceptile, switch it out, and THEN you can switch it into predicted Electric moves. Now, what kind of a competent player would spam Volt Switch when they know that the opponent has a M-Sceptile? That's right, nobody.
Besides, all Electric-types carry HP Ice because they pretty much have no other coverage. Jolteon, Thundurus, Rotom-W, you name it, it has HP Ice. If Sceptile can't OHKO them, it's dead. Simple as that. It sure as hell isn't countering any Electric-types anytime soon.
M-Sceptile also doesn't get Draco Meteor because it's not naturally a Dragon-type. Considering all these, M-Sceptile will be extremely mediocre Pokemon. It can be a good teammate for something that's extremely weak to Electric in Doubles/Triples, but that's it. I just don't see much reason to use your Mega slot on this thing. Going fast isn't going to be much if you can't do anything else.