I tested a more finely-tuned version of this archetype for Legacy with a friend. It can be a tough lock to get around. I can try to find our decklist if you would like.
If I had tested more, I could be sure of the details and be generally more thorough, but here is what I thought...
Against Survival decks: Scepter lockdown actually seems to have a slight (nothing amazing) advantage against what might be the best decks in the format. ATS finds it difficult playing a creature that can beat Morphling (or Exalted Angel if you can get it out). If the Survival deck can protect its SotF with Force of Will, it stands a good chance. If you can get a Scepter with a decent spell early on, your chances are even better.
Mono-blue/Forbidian/BBS/Whatever: Scepter lockdown only needs to resolve a 2 cc artifact. Blue control can easily go toe-to-toe with you in a counterwar, but it doesn't have a small spell that will win the game like that. Blue tends to do well against slow decks, but unless it can get appreciable card advantage, you are likely to come out on top.
Pox: Cursed Scroll will devastate you. Discard spells will hurt your ability to use countermagic. Pox can swing the game in its early stages. I never tested against anything like Pox. I am guessing, but I would not call it a good matchup. However, it has no way to deal with a Scepter, so things might be about even.
Combo: It should not be able to fight through your countermagic very well. Probably a decent matchup.
Aggro of any sort: Not insurmountable, but not particularly good. You can only deal with so many creatures in a short amount of time. If you can keep up a solid defense against creatures, time is usually on your side. The rapid beats that most aggro decks try to deal are rather unforgiving toward slow control decks.
Overall, I would not call this the best choice for Legacy in the current metagame. It has an edge against control, but not an impressive one. It is somewhat shaky against any aggro deck, which becomes a huge problem in this format (for now, at least). It has no matchups that are true autowins, and several where you are at a disadvantage.
Still, in a more casual setting, this thing's potent tech can become even more deadly. The decklist is missing the manabase (what is up with that happening so much around here?) and doesn't have enough countermagic. I would still imagine it would do well against most monoblack control decks (unless they're tuned or use broken cards or something). White Weenie could potentially outrace this deck, but it depends on the build...
