Quote from WaterdragonChains of Mephistopheles is a very specific card. I usually take out Sylvan Library for it as they don't work well together. So in that regard you have to ask the question: 'Which of the two can I benefit more from?'
These are my recommended match ups to board it in:
- Combo Elves; Glimpse of Nature, Regal Force and Elvish Visionary become unplayable, forcing them to turn into an aggro deck we can easily win from.
- Shardless BUG; Brainstorm and Ancestral Vision don't give them any advantage anymore, forcing them to go into a weak BUG control deck.
These are match ups where it's fair to board them in:
- Esper Stoneblade; it's ideal against Jace. the Mindsculptor, Brainstorm and Ponder, however you do feed Snapcaster Mage and Lingering Souls can be played with a detour.
- RUG Delver; while you do feed Nimble Mongoose and Tarmogoyf, both can be disarmed with a mirror Tarmogoyf, so your only concern would be Delver of Secrets and Grim Lavamancer who are easily taken care of with Punishing Fire. In this matchup I would rate Sylvan Library lower, as RUG can more efficiently play the cantrip game with Brainstorm and Ponder.
- ANT; shutting down cantrips such as Gitaxian Probe, Ponder and Preordain stalls them in finding their combo pieces. In this matchup it's better than having a Sylvan Library in play as you want to be more pro-actively disrupt their hand.
These are match ups where it's not recommended at all:
- Reanimate; you feed their graveyard. It's not good unless you also have an Extirpate to remove whatever hits their graveyard.
- Tempo BUG; you feed their graveyard again. While you match their Tarmogoyf with your own, Tombstalker hits the board earlier than normal. A 5/5 Flying creature is a huge problem for us. You don't want to run this card unless you have a Nihil Spellbomb right next to it.
In the end it depends heavily on your meta whether you need this card or not. In any case as a one off it's not so much game breaking or making; it's more about flavor and making certain match ups easier.